I'm not sure anyone is gonna make a third one at this rate. I keep hearing rumors about it, but it turns out to be another game. I just don't think there's enough AvP fans out there for anyone to take a game at it. If anything it'll most likely head to the PS3 or 360.
If anything does happen on the pc, I'm guessing they'll use the HL2 or Doom3 engine as both are easier to work with and more people will be able to run it. Such great graphics as the cry2 engine, come at a price, and only the crazy hardcore crowd would upgrade for just that type of game. I can see people rushing to upgrade for a game like Crysis, Doom, Quake, or HL but not for AvP, sadly. It's just not popular enough for anyone to take a stab at it. Maybe if AvP-R does well, that might convince Seirra or even Valve to make AvP3. If anything, I like to see Valve make AvP3.