Robert Carter has announced he'll be declaring and signing an agent, so he's as good as gone. Interesting move, though I think he's played himself into a mid first round pick, and probably about as high as he'll go.
Diamond Stone announced that he'll be declaring for the draft, but not hiring an agent just yet, so it appears he might see how things go at the NBA combine and then make a decision. My guess is that everything hinges on the combine, if he does well, he'll hire an agent and be gone. If scouts tell him stay in another year, I think he'll do that.
Melo Trimble hasn't said anything as far as I know. Rumors aside of him not going to class, I think it's more likely that he'll return than the other two. He clearly showed he's not ready for the NBA right now, and would likely not be drafted in the first round.