I agree that Ray may last the oldest (if not longest, since he is older than the other two). That all depends, though, on Ray staying here. The C's this year have really turned Ray into a Reggie Miller type player, coming off screens and firing jumpers. Sure, he does occasionally take it the hole, but not like he used to in his Seattle and Milwaukee days. That should go a long way in saving his legs and body. Pierce, on the other hand, routinely gets a beating taking it to the hole. Similarly, KG bangs down low.
On top of all of that, Ray may be in the best shape of all three. Pierce's weight can fluctuate from time to time and Garnett is probably too skinny for his own good.