If Sony actually decides to advertise Killzone 3, then KZ3 probably won't have to worry much about it's sales as much as Bulletstorm or Crysis 2.
Homefront won't do anything to alienate KZ3's sales. Ghost Recon is a TPS so it won't affect as much.
Crysis 2 does however seems like the biggest competition during Q1 2011 in terms of FPS.
EA is releasing Bulletstorm and Crysis 2 within a month's period so they pretty much have to choose which one to promote the hell out of and my guess is Crysis 2.
In fact EA has 3 Shooters releasing in Q1 2011.
Dead Space 2 in Jan, (Mass Effect 2 In Jan if you count it as Shooter/RPG Hybrid), Bulletstorm in Feb and Crysis 2 in March so, there's more of a chance that those games will eat each other's sales than affecting KZ3 by a noticable scale.