We discussed this briefly in the chat last night and I thought I'd open it up for discussion here.
First of all, I'm taking it as a given that Garnett is a Hall of Famer waiting to happen.
Here's my position on Pierce and Allen: I think thier fate is in their hands now: If the Celtics win a title with those two playing a prominant role on the team, then I think they are both in. Conversely, if they don't win a title with this group playing together (all healthy and other things being equal) then they will have squandered their best shot and probably don't deserve a place in the Hall. That's just my gut feeling right now (which could change 100 times) and I haven't done any extensive research into stats, etc.
If it is stats that you want, Basketball Reference uses some kind of calculation of stats to give a good thumbnail sketch of HOF candidates, but surely this can only be used as a very, very rough filter at best. I'm not even sure how up to date it is. But it made for some interesting comparisons.
Feel free to introduce your own stats, comparisons, and support for your opinions.