Halo 3 is one of those polarizing games in the annals of history. The Halo franchise was popular on the Xbox with Halo and Halo 2, then the jump into the next generation with the Xbox 360 was really when the world went nuts about Halo. This did a lot to strengthen the brand and also alienated some players who didn't like the attention and the influx of n00bs in the multiplayer. Halo is now one of the most-played franchises for multiplayer games in history and with good reason; it is awesome to play.
For some reason, there were some hints at Halo 3 maybe making the jump to Steam recently, but Microsoft has just quickly swatted that rumor down like the annoying fly that it was. Halo 3 remains an Xbox 360 exclusive, sorry PC guys. [Source]