Dieng and Withey are both surefire draft choices. Both have low ceiling but will be able to help right away especially defensively. I'd choose Whitey over Dieng just because Whitey looks a bit more beefy.
Steven Adams is a project, but a good one at that. He may not be able to protect the rim like the other two but he's already sound and a very good rebounder at this stage, and his upside is great.
BUT.
I'd really rather move up, trade the 16th, Avery Bradley and another guy and pick Shabazz Muhammad. All the talk about tanking for Andrew Wiggins when we can move up for this kid, who does have a ton of red flags when it comes to commitment and his game, but has a ridiculous upside, a very competitive nature and scoring abilities that is NBA ready. He can develop as an All Star in Boston. And if he for some reason did not work out, his upside could still land the Celtics good returns if we trade him.
Shabazz should be the target. He's dropping right now because of the red flags, his stock may rise again during the workouts as he is super athletic, but he is reachable.