Ming is already in his 5th season, and he has only gained 1 point this season. So he might gain one more point over the course of his career, but I don't see much more than that.
Watson is in his 3rd season, and he is a 21 year old DITR who is developing rapidly. He could turn out to be a mediocre ML pitcher someday, but he has a long way to go. I might give him a shot, but I wouldn't give very much for him.
Callaspo is also in his 3rd season, and is a 22 year old DITR. He is not developing nearly as rapidly as Watson, but he is 7 points ahead of him at this point. I'm guessing Callaspo and Watson will end up about the same in overall rating by the time they're done developing, probably somewhere in the upper 60's to low 70's. As brianp said, they will probably all end up as good AAA pitchers.