Crysis is 3 is not available on Steam. If you are looking to buy the game (the PC version), you can do that from Origin, EA’s gaming platform, similar to Steam.
You can order the game from here, the digital download version, for 69,99 euro.
Why is Crysis not available on Steam? Obviously because EA is trying to promote Origin and Steam is the competition.
However, we might see Crysis 3 on Steam in the future, considering that the previous two titles are there, Crysis 1 and 2.
Crysis 3 is the second important title the isn’t available on Steam, after Dead Space 3, another game developed by EA Games.