Interesting question, I'm interested in seeing how Bioware get around this since potentially all squad members could die in Mass Effect 2. Regardless, Bioware always stick old companions into sequels and I don't see why Mass Effect 3 would be any different. Given there were two returning companions in the second game, I reckon the same will be true of the third. For my guess as to who they would be, see below...
Personally I think the third game will be more focused on gathering all the races together in an epic battle against the Reapers as opposed to focusing on the squad members themselves, so I'd put money down on one of each race being viable squad members, so two Humans (the usual), an Asari, a Quarian, a Salarian, a Turian, a Krogan, a Drell etc. The reason I think this'll be the case is because...
Characters I highly doubt will be squad members are Thane, Zaeed (he's downloadable, it just seems like an odd choice) and Wrex (because of the spoiler tag above).
Liara and either Kaiden or Ashley. Purely because they're old companions who are garaunteed to survive through to the third game, plus it's alluded that forming a relationship in Mass Effect 2 may affect your old love interest in 3. It would be interesting if it created tension in the squad...
Personally I think the third game will be more focused on gathering all the races together in an epic battle against the Reapers as opposed to focusing on the squad members themselves, so I'd put money down on one of each race being viable squad members, so two Humans (the usual), an Asari, a Quarian, a Salarian, a Turian, a Krogan, a Drell etc. The reason I think this'll be the case is because...
It makes sense. The whole series so far has done this subtley, but you're given the option to form subtle allies with the other races. Wrex surviving may give you a better chance at allying with the Krogan, the Rachni promise to help "clear the darkness when the time comes" if you save the Queen, you can reform the Geth to becoming good, Liara has a lot of power and respect within the Asari community, helping the Migrant Fleet when in need with Tali's quest and, on a smaller scale, helping Thane to connect with his son may give you some sway on either the Drell community or Thane's son himself. Maybe he's a potential squad mate. Hmmm, that'd be interesting...
Characters I highly doubt will be squad members are Thane, Zaeed (he's downloadable, it just seems like an odd choice) and Wrex (because of the spoiler tag above).