v3.26.1
Label Element Value
Risk/Return: oef_RiskReturnAbstract  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Jan. 31, 2026
Registrant Name dei_EntityRegistrantName MFS® SERIES TRUST III
Entity Central Index Key dei_EntityCentralIndexKey 0000225604
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate May 28, 2026
Document Effective Date dei_DocumentEffectiveDate May 29, 2026
Entity Investment Company Type dei_EntityInvCompanyType N-1A
Prospectus Date oef_ProspectusDate May 29, 2026
MFS Global High Yield Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOAX
MFS Global High Yield Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOBX
MFS Global High Yield Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOCX
MFS Global High Yield Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOIX
MFS Global High Yield Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHORX
MFS Global High Yield Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOSX
MFS Global High Yield Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOTX
MFS Global High Yield Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOUX
MFS Global High Yield Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHOVX
MFS High Income Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHITX
MFS High Income Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHIBX
MFS High Income Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHICX
MFS High Income Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHIIX
MFS High Income Fund | Class R1  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHIGX
MFS High Income Fund | Class R2  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MIHRX
MFS High Income Fund | Class R3  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHIHX
MFS High Income Fund | Class R4  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHIJX
MFS High Income Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MHIKX
MFS High Yield Pooled Portfolio | Fund Shares  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol HYPPX
MFS Municipal High Income Fund | Class A  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MMHYX
MFS Municipal High Income Fund | Class B  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MMHBX
MFS Municipal High Income Fund | Class C  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MMHCX
MFS Municipal High Income Fund | Class I  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MMIIX
MFS Municipal High Income Fund | Class R6  
Risk/Return: oef_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MMHKX