There's no way it's Team Fortress 3. TF2 is one of the most played FPS games right now, it's STILL selling, and they STILL have updates coming for it. Since it's not story based like Left 4 Dead, there's no real sense behind making a sequel instead of just updating the current game, especially since it can't possibly meet or surpass TF2's popularity anytime soon, just divide the player base.
As for Left 4 Dead 3, I doubt it's that either, after the reaction they got when L4D2 was first announced. They'll probably wait a while before they do that one.
My guess?
1. DotA. No question here.
2. No HL2Ep3, at least not yet. Normally this wouldn't be a surprise, but since they said they have a surprise coming, many people will expect this game, which means that the status quo will actually come as a surprise to some.
3. Telltale Texas Hold'Em 2, featuring The Heavy.
P.S. Thanks