arma3launcher.exe / arma3battleye.exe with commandline to bypass launcher still prevents freesync.
If the game is started through arma3.exe amd crimson software can be used to turn freesync on and off with realtime results ingame.
If started through the launcher this action has no effect ingame (still jaggies).
Make sure that you run Arma 3 in fullscreen if you are using windows 10 since vsync will be forced on otherwise.