v3.26.1
Form N-1A Cover
Jan. 31, 2026
Prospectus [Line Items]  
Document Type 485BPOS
Amendment Flag false
Registrant Name MFS® SERIES TRUST III
Entity Central Index Key 0000225604
Entity Investment Company Type N-1A
Document Period End Date Jan. 31, 2026
Prospectus Date May 29, 2026