In some games where it's purely for eye candy, you can switch PhysX off completely. In others it performs some core functions plus eye candy, and only the latter can be switched off. Arma 3 will definitely use PhysX for vehicles, ragdolls and objects such as crates. There may also be additional eye candy that otherwise doesn't affect gameplay. I think it's highly unlikely they'd allow anything but the eye candy to be switched off, if it's implemented, especially given that it'd break multiplayer compatibility.