Dead Space 3 was released for PC and can be bought only from Origin (the EA Games gaming platform, similar to Steam).
Only the previous two titles are available on Steam, Dead Space 1 and 2.
You can buy the game from here, the PC digital download version.
I have tried the game and it looks interesting. It is at least an option until Aliens: Colonial Marines will be released.
DS3 doesn’t look very scary, the atmosphere is missing, it is more technical and with many casual elements. The control is poor, you have to press “Shift” to run, use melee attacks and some underpowered weapons. And the graphics.. not very stylish for a SF video game.
I don’t recommend this game.