RB3 will be a player who gives us something on ST, probably.
Dunbar's an interesting case, because he's both an injury risk and a player who gives you something valuable schematically. We might well be able to use Zeke the way we started using Dunbar last year--flexing him out and throwing against run defense personnel. But if Zeke were to miss time for any reason, keeping Dunbar and Morris or Jackson let's us keep that wrinkle in our offense.
Sure, we could do the same thing with McFadden in Morris' place, but I think Morris is safer by virtue of his 2 year contract this season.
If nobody plays ST well enough, I question the roster spot at all, and would rather keep three with one of them Morris (who doesn't miss time) and try to get Jackson to the PS. Dunbar, at least, has some moderate return values in STs.