Arma 3 will be exclusive to Steam, according to project lead Joris-Jan van't Land. He says that the goal as set by the Bohemia Interactive CEO is to release the game in 2013, and feels that "without going Steam-exclusive, we would not be able to release Arma 3 in 2013."
In a lengthy dev blog post, van't Land says that the Steam DRM and distribution network will help the company, while touting features like automatic patching to make the experience smoother for players. You'll need to run the game through Steam, but it will support an Offline mode. The post also notes that boxed copies will be available in most regions, but they'll simply be Steam codes in a retail package or possibly contain data to speed up the installation. Finally, van't Land suggests "there was a very real chance there would be no Arma 3 this year" without the Steam support, so even without the features he feels it's worthwhile to hit their goal.