I don't see a trade being realistic at this point. I can't see the Big 3 having any reasonable trade value whatsoever.
MAYBE Ray Allen; I keep picturing him as the last piece in Chicago, the assassin who gets them past Miami. Not sure what they might offer up in trade, but if anyone's got value, it's him.
Pierce could still have value, but he's got more value to the Celtics franchise long-term if he retires in green. They want Pierce around in twenty years, coming to games, getting a standing O from the crowd, being the face of this Celtics era. No messy divorce.
KG; nothing. He's basically done, but I'm not sure whether he knows it or not. He got all up in Dirk's grill on that last play, and Dirk just went right around him the other way. He's lost a step defensively, and his scoring and rebounding is obviously down a lot as well. Honestly, I think if he was under contract for next year as well, I think they might have amnestied him.
If we have good, young, valuable pieces to offer in a trade, KG could be useful to make the numbers work, but we're not trading for a guy like Dwight Howard.
On his own, KG's just not worth the money he's making, I don't see him as a guy who'll easily transition to a bench role, and I don't see him as a guy that contending teams are going to view as their last piece.