The developers have announced steam will be needed to run the game.

If you don't know what arma 3 is it's an open world tactical shooter video game being developed by Bohemia Interactive. It is expected to be released in Q3 2013. It's basically a open military sandbox where you can create your own gametypes or play pre-set ones like cap the flag. ARMA 3's campaign storyline takes place in the mid-2030s during the fictional Operation Magnitude, a military operation launched by NATO forces fighting in Europe against "Eastern armies" led by a resurgent Iran. The game is still in alpha stage where players who pre-ordered it get access to help play through the game and report bugs n' such.