Of the 16 squadmates you can have so far, only 1 is guaranteed to be in ME3, and that's Liara, seeing as how she's the only one with no possibility of dying.
Characters I'd like to see back would be:
Tali & Legion - The banter is great, the Quarian/Geth storyline is probably the most interesting of then all, also those 2 plus engineer Shepard = 3 COMBAT DRONES! (I should have picked this setup on Insane, seriously having 6 bodies in a fight helps).
Jack & Mordin - Jack because she holds 2 of my favourite character types, the psycho ***** and she has Survivor's Guilt. Mordin because his behaviour about the Genophage makes him more than just the stereotypical crazy scientist, shows his conflicting thoughts about what he's done, plus he's probably the most well written character out of all of the ME2 cast.
Ashley - Her human-first attitude in the first game combined with her Cerberus-goes-too-far attitude from the second make for an interesting final product.
Characters I'm ambivalent towards (in other words I wouldn't mind seeing back but won't be too bummed if they aren't) - Samara, Thane, Garrus, Grunt/Wrex (by now they're pretty much just palette swaps).
Characters I definitely don't want to see back:
Jacob & Miranda - Neither one is greatly likeable, Jacob is the simple (farm)soldier boy, and Miranda is the generic stuck-up *****, her sister storyline was phoned in and failed to garner any sort of real emotional or intellectual interest from me.
Kaiden - In every game of ME1 I ever play, I will always, always murder him. He's just a walking pile of 'meh'.
Zaeed - Fuck this guy, good but not great in a fight, and just a bitter old man with an uninteresting past, Wrex was better at being the grizzly veteran badass. Hell, Anderson was better at being the grizzly veteran badass.