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<!-- Generated by DFIN XBRL Schema Document - http://www.dfinsolutions.com/ - on 17-03-2026 05:57:04 PM -->
<!-- Copyright(c) 2026 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_OtherCategory2Domain" xlink:label="OtherCategory2Domain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/LargestHoldingOnly">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_LargestHoldingsDomain" xlink:label="LargestHoldingsDomain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/SalesLoadOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_WithSalesLoadMember" xlink:label="WithSalesLoadMember"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="InvestmentTypeCategorizationMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_NonFixedIncomeMember" xlink:label="NonFixedIncomeMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="NonFixedIncomeMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FixedIncomeSecuritiesMember" xlink:label="FixedIncomeSecuritiesMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="FixedIncomeSecuritiesMember" order="2.0"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMember" xlink:label="EquityMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="EquityMember" order="3.0"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="MoneyMarketFundsMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="MoneyMarketFundsMember" order="4.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/BroadBasedIndexOnly">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_BroadBasedIndexDomain" xlink:label="BroadBasedIndexDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergUSAggregateBondIndexMember" xlink:label="BloombergUSAggregateBondIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BroadBasedIndexDomain" xlink:to="BloombergUSAggregateBondIndexMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergGlobalAggregateIndexMember" xlink:label="BloombergGlobalAggregateIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BroadBasedIndexDomain" xlink:to="BloombergGlobalAggregateIndexMember" order="2.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergMunicipalBondIndexMember" xlink:label="BloombergMunicipalBondIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BroadBasedIndexDomain" xlink:to="BloombergMunicipalBondIndexMember" order="3.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/AdditionalIndexOnly">              
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_AdditionalIndexDomain" xlink:label="AdditionalIndexDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergUSCorporateHighYield2IssuerCappedIndexMember" xlink:label="BloombergUSCorporateHighYield2IssuerCappedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="BloombergUSCorporateHighYield2IssuerCappedIndexMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" xlink:label="ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" order="2.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/MaturityOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_MaturityDomain" xlink:label="MaturityDomain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/IndustrySectorOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_IndustrySectorDomain" xlink:label="IndustrySectorDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_TaxOtherMember" xlink:label="TaxOtherMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="TaxOtherMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_SalesAndExciseTaxRevenueMember" xlink:label="SalesAndExciseTaxRevenueMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="SalesAndExciseTaxRevenueMember" order="2.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_UniversitiesDormitoriesMember" xlink:label="UniversitiesDormitoriesMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="UniversitiesDormitoriesMember" order="3.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_UniversitiesCollegesMember" xlink:label="UniversitiesCollegesMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="UniversitiesCollegesMember" order="4.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_AirportRevenueMember" xlink:label="AirportRevenueMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="AirportRevenueMember" order="5.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_MultiFamilyHousingRevenueMember" xlink:label="MultiFamilyHousingRevenueMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="MultiFamilyHousingRevenueMember" order="6.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_MiscellaneousRevenueOtherMember" xlink:label="MiscellaneousRevenueOtherMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="MiscellaneousRevenueOtherMember" order="7.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_SecondarySchoolsMember" xlink:label="SecondarySchoolsMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="SecondarySchoolsMember" order="8.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_HealthcareRevenueHospitalsMember" xlink:label="HealthcareRevenueHospitalsMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="HealthcareRevenueHospitalsMember" order="9.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_HealthcareRevenueLongTermCareMember" xlink:label="HealthcareRevenueLongTermCareMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="IndustrySectorDomain" xlink:to="HealthcareRevenueLongTermCareMember" order="10.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/CreditQualityOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_CreditQualityDomain" xlink:label="CreditQualityDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_MoneyMarketFundsQualityMember" xlink:label="MoneyMarketFundsQualityMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="CreditQualityDomain" xlink:to="MoneyMarketFundsQualityMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_NotRatedMember" xlink:label="NotRatedMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="CreditQualityDomain" xlink:to="NotRatedMember" order="2.0"/>
				</link:definitionLink>
				<!--GeographicRegionOnly use only for STPR-->
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/GeographicRegionOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_GeographicRegionDomain" xlink:label="GeographicRegionDomain"/>
				</link:definitionLink>
				<link:labelLink xlink:type="extended" xlink:role="http://www.xbrl.org/2003/role/link">               
					<link:loc xlink:type="locator" xlink:href="#mstiii_NonFixedIncomeMember" xlink:label="NonFixedIncomeMember"/>
					<link:label xlink:type="resource" xlink:label="label_NonFixedIncomeMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Non Fixed Income [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="NonFixedIncomeMember" xlink:to="label_NonFixedIncomeMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_MoneyMarketFundsQualityMember" xlink:label="MoneyMarketFundsQualityMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoneyMarketFundsQualityMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Money Market Funds Quality [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoneyMarketFundsQualityMember" xlink:to="label_MoneyMarketFundsQualityMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_NotRatedMember" xlink:label="NotRatedMember"/>
					<link:label xlink:type="resource" xlink:label="label_NotRatedMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Not Rated [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="NotRatedMember" xlink:to="label_NotRatedMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergUSCorporateHighYield2IssuerCappedIndexMember" xlink:label="BloombergUSCorporateHighYield2IssuerCappedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergUSCorporateHighYield2IssuerCappedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg U S Corporate High Yield 2 Issuer Capped Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergUSCorporateHighYield2IssuerCappedIndexMember" xlink:to="label_BloombergUSCorporateHighYield2IssuerCappedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergUSAggregateBondIndexMember" xlink:label="BloombergUSAggregateBondIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergUSAggregateBondIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg U S Aggregate Bond Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergUSAggregateBondIndexMember" xlink:to="label_BloombergUSAggregateBondIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000116932Member" xlink:label="C000116932Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000116932Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000116932 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000116932Member" xlink:to="label_C000116932Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000116932Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R6</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000116932Member" xlink:to="label_C000116932Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006472Member" xlink:label="C000006472Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006472Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006472 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006472Member" xlink:to="label_C000006472Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006472Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R4</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006472Member" xlink:to="label_C000006472Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006483Member" xlink:label="C000006483Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006483Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006483 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006483Member" xlink:to="label_C000006483Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006483Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R3</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006483Member" xlink:to="label_C000006483Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006482Member" xlink:label="C000006482Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006482Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006482 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006482Member" xlink:to="label_C000006482Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006482Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R2</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006482Member" xlink:to="label_C000006482Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006480Member" xlink:label="C000006480Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006480Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006480 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006480Member" xlink:to="label_C000006480Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006480Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R1</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006480Member" xlink:to="label_C000006480Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006478Member" xlink:label="C000006478Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006478Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006478 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006478Member" xlink:to="label_C000006478Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006478Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class I</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006478Member" xlink:to="label_C000006478Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006477Member" xlink:label="C000006477Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006477Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006477 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006477Member" xlink:to="label_C000006477Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006477Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class C</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006477Member" xlink:to="label_C000006477Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006476Member" xlink:label="C000006476Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006476Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006476 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006476Member" xlink:to="label_C000006476Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006476Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class B</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006476Member" xlink:to="label_C000006476Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006471Member" xlink:label="C000006471Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006471Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006471 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006471Member" xlink:to="label_C000006471Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006471Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class A</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006471Member" xlink:to="label_C000006471Member_2"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FixedIncomeSecuritiesMember" xlink:label="FixedIncomeSecuritiesMember"/>
					<link:label xlink:type="resource" xlink:label="label_FixedIncomeSecuritiesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fixed Income Securities [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FixedIncomeSecuritiesMember" xlink:to="label_FixedIncomeSecuritiesMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMember" xlink:label="EquityMember"/>
					<link:label xlink:type="resource" xlink:label="label_EquityMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Equity [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="EquityMember" xlink:to="label_EquityMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="MoneyMarketFundsMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoneyMarketFundsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Money Market Funds [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoneyMarketFundsMember" xlink:to="label_MoneyMarketFundsMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchCCRatingMember" xlink:label="FitchCCRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchCCRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, CC Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchCCRatingMember" xlink:to="label_FitchCCRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchCCCRatingMember" xlink:label="FitchCCCRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchCCCRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, CCC Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchCCCRatingMember" xlink:to="label_FitchCCCRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchBRatingMember" xlink:label="FitchBRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchBRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, B Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchBRatingMember" xlink:to="label_FitchBRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchBBRatingMember" xlink:label="FitchBBRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchBBRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, BB Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchBBRatingMember" xlink:to="label_FitchBBRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchBBBRatingMember" xlink:label="FitchBBBRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchBBBRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, BBB Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchBBBRatingMember" xlink:to="label_FitchBBBRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_BR" xlink:label="BR"/>
					<link:label xlink:type="resource" xlink:label="label_BR" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">BRAZIL</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BR" xlink:to="label_BR"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CA" xlink:label="CA"/>
					<link:label xlink:type="resource" xlink:label="label_CA" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">CANADA</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="CA" xlink:to="label_CA"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_FR" xlink:label="FR"/>
					<link:label xlink:type="resource" xlink:label="label_FR" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">FRANCE</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FR" xlink:to="label_FR"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IN" xlink:label="IN"/>
					<link:label xlink:type="resource" xlink:label="label_IN" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">INDIA</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="IN" xlink:to="label_IN"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IT" xlink:label="IT"/>
					<link:label xlink:type="resource" xlink:label="label_IT" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">ITALY</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="IT" xlink:to="label_IT"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_LU" xlink:label="LU"/>
					<link:label xlink:type="resource" xlink:label="label_LU" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">LUXEMBOURG</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="LU" xlink:to="label_LU"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_MX" xlink:label="MX"/>
					<link:label xlink:type="resource" xlink:label="label_MX" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MEXICO</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MX" xlink:to="label_MX"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB" xlink:label="GB"/>
					<link:label xlink:type="resource" xlink:label="label_GB" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">UNITED KINGDOM</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="GB" xlink:to="label_GB"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US" xlink:label="US"/>
					<link:label xlink:type="resource" xlink:label="label_US" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">UNITED STATES</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="US" xlink:to="label_US"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MoodysCRatingMember" xlink:label="MoodysCRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoodysCRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Moody&apos;s, C Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoodysCRatingMember" xlink:to="label_MoodysCRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchARatingMember" xlink:label="FitchARatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchARatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, A Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchARatingMember" xlink:to="label_FitchARatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchDRatingMember" xlink:label="FitchDRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchDRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, D Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchDRatingMember" xlink:to="label_FitchDRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006484Member" xlink:label="C000006484Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006484Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006484 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006484Member" xlink:to="label_C000006484Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006485Member" xlink:label="C000006485Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006485Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006485 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006485Member" xlink:to="label_C000006485Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006486Member" xlink:label="C000006486Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006486Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006486 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006486Member" xlink:to="label_C000006486Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006487Member" xlink:label="C000006487Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006487Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006487 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006487Member" xlink:to="label_C000006487Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000064677Member" xlink:label="C000064677Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000064677Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000064677 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000064677Member" xlink:to="label_C000064677Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000064678Member" xlink:label="C000064678Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000064678Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000064678 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000064678Member" xlink:to="label_C000064678Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000064679Member" xlink:label="C000064679Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000064679Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000064679 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000064679Member" xlink:to="label_C000064679Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000064680Member" xlink:label="C000064680Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000064680Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000064680 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000064680Member" xlink:to="label_C000064680Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000064681Member" xlink:label="C000064681Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000064681Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000064681 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000064681Member" xlink:to="label_C000064681Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergGlobalAggregateIndexMember" xlink:label="BloombergGlobalAggregateIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergGlobalAggregateIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg Global Aggregate Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergGlobalAggregateIndexMember" xlink:to="label_BloombergGlobalAggregateIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" xlink:label="ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember"/>
					<link:label xlink:type="resource" xlink:label="label_ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">ICE BofA Global High Yield Constrained Index USD Hedged [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" xlink:to="label_ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_OtherCountriesMember" xlink:label="OtherCountriesMember"/>
					<link:label xlink:type="resource" xlink:label="label_OtherCountriesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Other Countries [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="OtherCountriesMember" xlink:to="label_OtherCountriesMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_TaxOtherMember" xlink:label="TaxOtherMember"/>
					<link:label xlink:type="resource" xlink:label="label_TaxOtherMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Tax Other [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="TaxOtherMember" xlink:to="label_TaxOtherMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_SalesAndExciseTaxRevenueMember" xlink:label="SalesAndExciseTaxRevenueMember"/>
					<link:label xlink:type="resource" xlink:label="label_SalesAndExciseTaxRevenueMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Sales And Excise Tax Revenue [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="SalesAndExciseTaxRevenueMember" xlink:to="label_SalesAndExciseTaxRevenueMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_UniversitiesDormitoriesMember" xlink:label="UniversitiesDormitoriesMember"/>
					<link:label xlink:type="resource" xlink:label="label_UniversitiesDormitoriesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Universities Dormitories [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="UniversitiesDormitoriesMember" xlink:to="label_UniversitiesDormitoriesMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_UniversitiesCollegesMember" xlink:label="UniversitiesCollegesMember"/>
					<link:label xlink:type="resource" xlink:label="label_UniversitiesCollegesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Universities Colleges [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="UniversitiesCollegesMember" xlink:to="label_UniversitiesCollegesMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_AirportRevenueMember" xlink:label="AirportRevenueMember"/>
					<link:label xlink:type="resource" xlink:label="label_AirportRevenueMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Airport Revenue [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="AirportRevenueMember" xlink:to="label_AirportRevenueMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_MultiFamilyHousingRevenueMember" xlink:label="MultiFamilyHousingRevenueMember"/>
					<link:label xlink:type="resource" xlink:label="label_MultiFamilyHousingRevenueMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Multi Family Housing Revenue [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MultiFamilyHousingRevenueMember" xlink:to="label_MultiFamilyHousingRevenueMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_MiscellaneousRevenueOtherMember" xlink:label="MiscellaneousRevenueOtherMember"/>
					<link:label xlink:type="resource" xlink:label="label_MiscellaneousRevenueOtherMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Miscellaneous Revenue Other [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MiscellaneousRevenueOtherMember" xlink:to="label_MiscellaneousRevenueOtherMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_SecondarySchoolsMember" xlink:label="SecondarySchoolsMember"/>
					<link:label xlink:type="resource" xlink:label="label_SecondarySchoolsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Secondary Schools [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="SecondarySchoolsMember" xlink:to="label_SecondarySchoolsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_HealthcareRevenueHospitalsMember" xlink:label="HealthcareRevenueHospitalsMember"/>
					<link:label xlink:type="resource" xlink:label="label_HealthcareRevenueHospitalsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Healthcare Revenue Hospitals [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="HealthcareRevenueHospitalsMember" xlink:to="label_HealthcareRevenueHospitalsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_HealthcareRevenueLongTermCareMember" xlink:label="HealthcareRevenueLongTermCareMember"/>
					<link:label xlink:type="resource" xlink:label="label_HealthcareRevenueLongTermCareMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Healthcare Revenue Long Term Care [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="HealthcareRevenueLongTermCareMember" xlink:to="label_HealthcareRevenueLongTermCareMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_BloombergMunicipalBondIndexMember" xlink:label="BloombergMunicipalBondIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergMunicipalBondIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg Municipal Bond Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergMunicipalBondIndexMember" xlink:to="label_BloombergMunicipalBondIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000191232Member" xlink:label="C000191232Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000191232Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000191232 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000191232Member" xlink:to="label_C000191232Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000103072Member" xlink:label="C000103072Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000103072Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000103072 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000103072Member" xlink:to="label_C000103072Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006490Member" xlink:label="C000006490Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006490Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006490 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006490Member" xlink:to="label_C000006490Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006489Member" xlink:label="C000006489Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006489Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006489 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006489Member" xlink:to="label_C000006489Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000006488Member" xlink:label="C000006488Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006488Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006488 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006488Member" xlink:to="label_C000006488Member"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchAARatingMember" xlink:label="FitchAARatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchAARatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, AA Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchAARatingMember" xlink:to="label_FitchAARatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MoodysAaaRatingMember" xlink:label="MoodysAaaRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoodysAaaRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Moody&apos;s, Aaa Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoodysAaaRatingMember" xlink:to="label_MoodysAaaRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstiii_C000118060Member" xlink:label="C000118060Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000118060Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000118060 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000118060Member" xlink:to="label_C000118060Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000118060Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class *</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000118060Member" xlink:to="label_C000118060Member_2"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeAxis" xlink:label="InvestmentTypeAxis"/>
					<link:label xlink:type="resource" xlink:label="label_InvestmentTypeAxis" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Investment Type [Axis]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="InvestmentTypeAxis" xlink:to="label_InvestmentTypeAxis" priority="1"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsNet" xlink:label="AssetsNet"/>
					<link:label xlink:type="resource" xlink:label="label_AssetsNet" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Net Assets</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="AssetsNet" xlink:to="label_AssetsNet" priority="1"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyPortfolioTurnover" xlink:label="InvestmentCompanyPortfolioTurnover"/>
					<link:label xlink:type="resource" xlink:label="label_InvestmentCompanyPortfolioTurnover" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Investment Company Portfolio Turnover</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="InvestmentCompanyPortfolioTurnover" xlink:to="label_InvestmentCompanyPortfolioTurnover" priority="1"/>                
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_NoTradingSymbolFlag" xlink:label="dei_NoTradingSymbolFlag"/>
					<link:label xlink:type="resource" xlink:label="label_NoTradingSymbolFlag" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">No Trading Symbol Flag</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_NoTradingSymbolFlag" xlink:to="label_NoTradingSymbolFlag" priority="1"/>		
				</link:labelLink>
			</link:linkbase>
		</appinfo>
	</annotation>   
	<element name="NonFixedIncomeMember" id="mstiii_NonFixedIncomeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="MoneyMarketFundsQualityMember" id="mstiii_MoneyMarketFundsQualityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="NotRatedMember" id="mstiii_NotRatedMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergUSCorporateHighYield2IssuerCappedIndexMember" id="mstiii_BloombergUSCorporateHighYield2IssuerCappedIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergUSAggregateBondIndexMember" id="mstiii_BloombergUSAggregateBondIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000116932Member" id="mstiii_C000116932Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006472Member" id="mstiii_C000006472Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006483Member" id="mstiii_C000006483Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006482Member" id="mstiii_C000006482Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006480Member" id="mstiii_C000006480Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006478Member" id="mstiii_C000006478Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006477Member" id="mstiii_C000006477Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006476Member" id="mstiii_C000006476Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006471Member" id="mstiii_C000006471Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006484Member" id="mstiii_C000006484Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006485Member" id="mstiii_C000006485Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006486Member" id="mstiii_C000006486Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006487Member" id="mstiii_C000006487Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000064677Member" id="mstiii_C000064677Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000064678Member" id="mstiii_C000064678Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000064679Member" id="mstiii_C000064679Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000064680Member" id="mstiii_C000064680Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000064681Member" id="mstiii_C000064681Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergGlobalAggregateIndexMember" id="mstiii_BloombergGlobalAggregateIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" id="mstiii_ICEBofAGlobalHighYieldConstrainedIndexUSDHedgedMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="OtherCountriesMember" id="mstiii_OtherCountriesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="TaxOtherMember" id="mstiii_TaxOtherMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="SalesAndExciseTaxRevenueMember" id="mstiii_SalesAndExciseTaxRevenueMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="UniversitiesDormitoriesMember" id="mstiii_UniversitiesDormitoriesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="UniversitiesCollegesMember" id="mstiii_UniversitiesCollegesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="AirportRevenueMember" id="mstiii_AirportRevenueMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="MultiFamilyHousingRevenueMember" id="mstiii_MultiFamilyHousingRevenueMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="MiscellaneousRevenueOtherMember" id="mstiii_MiscellaneousRevenueOtherMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="SecondarySchoolsMember" id="mstiii_SecondarySchoolsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="HealthcareRevenueHospitalsMember" id="mstiii_HealthcareRevenueHospitalsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="HealthcareRevenueLongTermCareMember" id="mstiii_HealthcareRevenueLongTermCareMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergMunicipalBondIndexMember" id="mstiii_BloombergMunicipalBondIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000191232Member" id="mstiii_C000191232Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000103072Member" id="mstiii_C000103072Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006490Member" id="mstiii_C000006490Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006489Member" id="mstiii_C000006489Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006488Member" id="mstiii_C000006488Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000118060Member" id="mstiii_C000118060Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
</schema>
