In October 2020, Martins joined ART for the post-season test at Catalunya, which saw him finish in the top ten across all four sessions, with a highest placed finish of fifth on the first day. He ended up signing for MP Motorsport for the 2021 Formula 3 season, alongside fellow Alpine junior and Formula Renault Eurocup title rival Caio Collet and Tijmen van der Helm. The Frenchman's season started out positively, after a second place in race two of the opening round, and two podiums at his home round in Le Castellet. However, after a fifth place finish at the first race in Austria, Martins went five races without scoring points.