v3.25.3
First Trust Merger Arbitrage ETF N-1A Cover
Jul. 31, 2025
Prospectus [Line Items]  
Document Type 485BPOS
Amendment Flag false
Registrant Name First Trust Exchange-Traded Fund III
Entity Central Index Key 0001424212
Entity Investment Company Type N-1A
Document Period End Date Jul. 31, 2025
Prospectus Date Dec. 01, 2025