Amos is the starter, and I don't Bettis will be there much longer. I wouldn't be suprised to see Bettis in Tampa or Dallas soon.
Smith and Barlow are coaches favorites. 9er coaches are saying that Barlow's blocking is better and they won't hesitate to use him on 3rd downs this year.
All that said, right now, I would go Amos, Barlow, Smith in that order.