Is it possible that I'm playing against players on the PS3, Xbox and the Mac in a single game?
Or does each platform have their own servers and games?
No, each platform has its own servers. You are always playing against players on the same platform. The matchmaking systems are all separated and they don't communicate at all.
The Xbox and PS3 both have proprietary online systems (Live and PSN, respectively) that are generally walled off from each other for most games, including all the Call of Duty games.
In particular, the PC players using a mouse and keyboard tend to destroy console players - this has been a reason why they have been separated in other games. One developer talked about the differences that they had to make when porting their game to PC over on Penny Arcade.
It's possible that Mac and PC gamers could use the same servers; I don't know how likely this is.