I think that they are for PC, I just forgot. :P
Im not sure about Halo 3 though.
Halo 3 they have not made for PC and Halo 2 i think is supposed to come out for PC soon because they just now made it for PC
Halo 3 is only on 360. It'll come out on PC around 2011 or so probaly.
yeah knowing how long it took Bungie to make Halo 2 for the PC