I'd just throw out a warning: Arma 3 is roughly 900% improved over Arma 2 in every metric except sheer amount of content. The older games are good for figuring out arma's byzantine controls, how the editor works, and getting a feel for the general game mechanics. However the way your character moves, the way the AI behave, the way your character handles a weapon; all of these things and more have changed drastically for the better in Arma 3.
I wouldn't discourage you from picking up Arma 2, it's still a great game. Just be aware that it is not a 1:1 reflection for how Arma 3 handles.