So the votes are in and I agree with the majority. To nobody's surprise, the best Celtics 3 on 3 team would be Rondo, Pierce, and Garnett. (The rules of this particular exercise didn't allow for a sub - so no Terry or Bradley or Green or Bass to rotate in)
Allow me to make the case for how they would dominate by describing a typical offensive set.
Rondo holds the ball at the top of the key, surveying the scene with his "I'm playing chess while these jokers are playing checkers" look. Pierce is stationed at the 3 point line just beyond the 45 degree spot, where he loves to catch and either shoot or start his slow-motion-but-you-have-to-foul-me-to-stop-me dribble drive routine. Garnett is at the elbow ready to set a pick or flash out to his sweet spot just inside the 3 point line.
If Rondo's man is giving him a couple of feet he starts his drive right down the gut of the defense where he can break any defender down one-on-one. If anyone comes over to help, nobody is better at spotting the open man and hitting him with the ball than Rondo.
If Rondo's man plays up tight, Garnett sets a pick and rolls either to his sweet spot or to the basket for a lob over the top - depending on what his man does. This is particularly deadly since there's no weak side defending big man, and if Pierce's man comes to help that leaves him open. Did I mention yet that Pierce won the 3 point shooting contest a couple years ago?
You might also see Rondo toss the ball out to Pierce to start an offensive set with KG and Rajon finding open space and cutting depending on what Pierce's defender does. Or you could just toss it into the post and let KG embarrass Chris Bosh or most of the other big men that won't have their lumbering centers to help them bang with Garnett.
Of course a lot of teams will have similar offensive advantages and strategies, but perhaps the area where the Celtics would stand out the most would be on defense. Keep in mind, this "old" team will not have to run the break. They can conserve their energy for short bursts in the half court. Pierce has shown that he can be a lockdown guy for short bursts - even giving the LeBrons and Kobes of the world fits. Rondo is a gambler but he can be an excellent defender when he focuses on it. And of course Kevin Garnett is the field general, shouting out instructions and foaming at the mouth. Nobody defends the pick and roll better and this tourney would be all about the pick and roll.
As a bonus, this team is nasty, in a good way. Rondo is not giving any quarter to any guard, Pierce is a king of trash talk and Garnett is just a certifiable loon who's bark is on par with his "bite" (defensive chops). They will score on you, stop you, and let you know about it in no uncertain terms.
So there you have it. That sweet threesome would give any team a run for their money and might just have what it takes to take home the title. Not that there's any room in the rafters for tourney wins, but it would be a lot of fun to watch.