Just like Battlefield 3, Mass Effect 3 won't be available on Steam since EA listed the game going to just Origin and a few other providers.
The PC version also requires a constant internet connection for multiplayer, no limits on installs but does require a mandatory one time verification check.
No news if the saves from the previous games will work on Origin but a lot of people are assuming it won't be possible if Origin doesn't allow Steam stuff to be used.
http://kotaku.com/5876171/surprise-s...aunch-on-steam