Searching for a "free download" of the full version of typically leads to unofficial or pirated copies rather than legitimate free offers. Arma 3 is a paid, Steam-exclusive title and does not have a permanent free full version. Legitimate Ways to Play for Free

While the full game is not free, there are official ways to experience it or its content without paying upfront: Arma 3 Mods - How to play Arma 3 CDLC for FREE [Legit]