v3.26.1
Cover - USD ($)
 12 Months Ended
Dec. 31, 2025
Mar. 25, 2026
Jun. 30, 2025
Document Information [Line Items]      
Document Type 10-K    
Document Transition Report false    
Entity Interactive Data Current Yes    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Period End Date Dec. 31, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Entity Registrant Name HCM III ACQUISITION CORP.    
Entity Central Index Key 0002069856    
Entity File Number 001-42774    
Entity Tax Identification Number 98-1854444    
Entity Incorporation, State or Country Code E9    
Entity Current Reporting Status Yes    
Entity Shell Company true    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Address, Address Line One 85 Washington Street    
Entity Address, City or Town Norwalk    
Entity Address, State or Province CT    
Entity Address, Postal Zip Code 06902    
City Area Code (203)    
Local Phone Number 930-2200    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Public Float     $ 0
Document Financial Statement Error Correction [Flag] false    
ICFR Auditor Attestation Flag false    
Document Annual Report true    
Units, each consisting of one Class A ordinary share and one-third of one Redeemable Warrant      
Document Information [Line Items]      
Title of 12(b) Security Units, each consisting of one Class A ordinary share and one-third of one Redeemable Warrant    
Trading Symbol HCMAU    
Security Exchange Name NASDAQ    
Class A Ordinary Shares, par value $0.0001 per share      
Document Information [Line Items]      
Title of 12(b) Security Class A Ordinary Shares, par value $0.0001 per share    
Trading Symbol HCMA    
Security Exchange Name NASDAQ    
Redeemable Warrants, each whole warrant exercisable for one Class A ordinary share at a price of $11.50 per share      
Document Information [Line Items]      
Title of 12(b) Security Redeemable Warrants, each whole warrant exercisable for one Class A ordinary share at a price of $11.50 per share    
Trading Symbol HCMAW    
Security Exchange Name NASDAQ    
Class A Ordinary Shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   25,300,000  
Class B Ordinary Shares      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   8,433,333