With no one out and a runner on third base, a bunt is being used for the purpose of scoring a run. This bunt would have to be coming up the first base line because bunting up the third base line would be a huge risk. The worst case scenario for the base runner in this situation is if the ball is bunted too hard back at the pitcher or is popped up. If the runner is going hard trying to advance then they could get thrown out or doubled off. Again, the bunt would be less likely with one out and a lot less likely with two outs.
First Base: Charge in toward the bunt
Second Base: Go to cover first base
Short Stop: Go to cover third
Third Base: Charge in towards the bunt
Pitcher: Charge towards bunt
Catcher: Move towards the bunt
Outfielders: Charge in towards the infield to back up and cover bases