I actually think there is a good chance that we'll see all three back.
PP is still under contract.
Garnett says he wants to return, and he can still start, imo. This will depend on the money he's offered from other teams. He's the most likely to be gone in my mind, but there's a chance I think.
Allen will stay. He has a child with special medical needs and he has said that he's recieved the greatest care and attention in Boston, bar none. He won't give that up for another city I think. The question is do the C's still want him, and I think they do. Albeit as a bench player, but I think Allen is ready to accept that kind of role at this point in his career.
I don't think it will be the Big 3's team anymore - it will be Rondo's team. Put some talent around him (keep Bass if you can, I love him) and the team will have a new look. But the Big 3 can still be a part of it and I hope they will be.