Every (surviving) character from ME2 will have a part to play in ME3, that's been confirmed. Though only Garrus and Tali return as squad members.
Even if it hadn't been confirmed, they had a clip of Jacob in one of the trailers. If they include Jacob, then then it'd be almost certain everyone else (read: characters will some actual interest and personality) would show up also.
It makes sense that the band would all split up though. They're the single best minds in the galaxy, which is why you recruited them in ME2. To save a galaxy, you need people like that all over the place coordinating the defence/attack. Also the SR2 is re-appropriated by the Alliance Military, and the higher-ups would probably kick them all off if they haven't left before that.
At the same time, the fanboy in me would have loved to see them all at their stations on the Normandy, or moving around the ship and interacting with your new crew members.
As a side note - this probably doesn't belong here, but it's not worth making a new topic for - BW have unveiled the final version of their ME3 Liara figure. I have to say that it's a highly detailed, great looking figure... unfortunately, it looks nothing like Liara. Not to mention she seems to have had a boob job.Link
It's an oldish story, but I hadn't seen the "final" painted version until now.[Edited by Skyheart, 2/29/2012 6:42:25 AM]