Chauncy Billups for Kenny Anderson in hindsight was not good (though it did take several more years and teams for Chauncy to really develop). Although technically not a 3 team trade, it basically was because Portland traded Kenny Anderson to Toronto but Kenny refused to play there so he then got traded to Boston. So essentially a 3-team, 3-point guard trade in '97 involving Kenny Anderson, Chauncy Billups, and Damon Stoudamire, and from '97 on I think Kenny easily had the worst career of the three. Billups was a good trade chip that we should have gotten more for.
Eric Williams for two 2nd round picks after 2 really good seasons with the C's? That one still bothers me.
Joe Johnson's worth mentioning again, like Billups gave him away way too early, and for too little (although I was a big Rodney Rogers fan when he was here).
Trading away Sherman Douglas, who was the catalyst for the '95 playoff run (granted the team only won 35 games and lost in the 1st round, but 3 of the 4 playoff games went right down to the wire) for Todd Day was a mistake. Hey I understand the team just signed 5'11" Dana Barros and had 6'1" Dee Brown and young 6'0" David Wesley was looking promising and you don't really need four 6' guards on your team, but Sherman Douglas was the general the young Celtics needed.
Whether you love him or hate, you gotta admit Antoine for Raef (essentially) was like getting 10 cents on the dollar (although we did get a pick that ended up being Delonte so that evens it out a little). Actually to me, most of the mid-90's to mid-00's trades seem like we were getting back pennies on the dollar. Not a good time.