Year 1: Big 3 along with Doc win championship.
Year 2: It wasn't another team that beat them, it was an injury.
Year 3: They overcome a bad regular season, age, various injuries, and a brutal playoff opponent draw to fight their way to the finals... only to have to play a powerhouse Lakers team, heavily favored, led by one of the top 2 players in the league, coached by the winningest coach ever, and on top of even that the Lakers have the home court advantage. 
Celts get blown out in game 1, come back and win must win game 2 ON THE ROAD.  And then in Game 6 they lose Perk (that's a huge loss no matter what anyone says).

I'm a realist.  Let's face it, the big 3 along with Doc are unlikely to get to the finals again.  They are already showing serious signs of decline and they are likely to have injury problems in the future.  The Celts don't have a top 2 pick waiting in the wings like they did in '86.  So even if they manage to keep the team together (unlikely in and of itself), this is probably the last hurrah for these guys.

That means that their legacy is largely going to be decided by what happens tomorrow night.  Does it get any tougher than this?  Perk out, on the road, biased refs, Kobe Bryant, Phil Jackson...

I can only speak for myself.  These guys have nothing left to prove to me.  As far as I'm concerned Paul Pierce has proven himself a winner and I believe one of the top 6 or 7 Celtics ever.  Ray Allen is a much better player than I ever realized before he came to Boston.  He's one of those guys that you have to watch everyday to truly appreciate.  KG is a very unique player that is better than I thought he would be, too.  And Doc is a great coach... better than KC Jones or Bill Fitch.

BUT, I'm in minority and the casual fan will only remember the 'ordinary' 1 championship if they lose tomorrow, and that's a shame.  Can you imagine what we might have had if we had all 3 of these guys together in their prime?