Of those three, Fitz is the likely candidate to come off the bench this season. He's essentially a one-dimensional player who can score from fifteen and in. IMO, his defense and rebounding (or lack thereof) will make him the odd man out. At the same time, I agree with BOK in that Drew will give us a scoring option off the bench that we really haven't had in sometime.
No one mentioned Cam Clark, who was a starter in most of the games he participated in last year. He has to be considered in trying to determine who starts this season. If LK wants to play up tempo and press full court, it will be Osby, M'Baye and Clark. If he decides to go with more size, he'll start Fitz, Osby and M'Baye and let Cam come off the bench.
The best part about this discussion is that we have the depth to talk about bringing starters the past two seasons off the bench in a back up role this year. No matter who starts, I see that as a VERY good thing!