I for one would be more than happy if Gambit NEVER shows up in an X-Men film. The truth is, though, he might be in number 3 simply because he looks normal and his powers would be very simple to handle through special effects. Cable I would LOVE to see, alongside his X-Force strike team members like Domino (who has always been a personal favorite of mine), Shatterstar (cool character, lousy design), and Cannonball (as if Rogue wasn't southern fried enough). The sentinals would be great to see, and from what I am hearing they might indeed be appearing, but it is waaaay too soon to confirm anything like that.
I'd like to see Beast (I forget who brought him up), but it is doubtful. His selling points are his high IQ, his athletic prowess, and his bizarre appearance. With Nightcrawler we already have somebody who is athletic and weird looking, and Jean Grey seems to be the resident brain of the group (and yes, we all know that she will be popping up again in the next film).
Some characters I would personally like to see: Juggernaut (and his buddy Archie-naut; sorry, couldn't resist), Apocalypse, Deadpool, Collossus, Shadowcat (in a REAL role), Sabretooth, and Blob. I think that if they focus on a human-mutant war in the next film, we will see a LOT of Marvel's mutants, but only in brief cameos.
Some characters I DON'T want to see: Gambit, Maggot, Marrow, Celia Reyes, Black Tom, Pete Wisdom, any of those freaking loser Morlocks (well, maybe except for Caliban and Callisto), the X-Babies, Longshot, Dazzler, Starjammers (ie: X-Men: the Star Wars Rip-off), anybody with an "X" in their code name.