All three of these players had very successful college careers, but all three of them are projected to have much less success in the NBA. Although none of them will likely be big time contributors to the 2012-2013 Celtics' squad, Sullinger is the only one who will receive consistant minutes.
This was displayed in the Celtics' season opener against the Miami Heat as Sullinger was the only rookie to play for the Celtics. Both Joseph and Melo will likely spend portions of their rookie seasons playing for the Maine Red Claws in the NBA's Developmental League.
The Celtics selected two very different players in the first round.
Sullinger is going to be a good player, but doesn't have a very high ceiling as he has limited athletic ability. Fab Melo, on the other hand, is completely different, as he could either be a complete bust or a very good player.
The Celtics are used to getting little to no contributions from rookies in recent years, as they are almost always picking in the late first round.