Video: Danny Ainge discusses Celtics options after getting the #3 pick
Basically what you'd expect from Ainge. He'll look to trade the pick, but that's easier said than done. First you'd need to find a team willing to trade a caliber player worth giving up a top 3 pick for. And then said team would need to have a reasonable asking price for their player. My guess is the Ainge will keep the pick and draft the player most likely to have star potential.
Then Ainge will consolidate some of his other picks to move up for a 2nd player that he really likes. You're not going to be able to move up too much with the Mavs pick, the Celtics own first and their early 2nds, but you can leap a couple teams that also have eyes for a guy you like.