The 3rd basemen, 1st basemen and pitcher would charge to cover the bunt, the ss would cover 3rd and second basemen would cover 1st. the reason why is simple, bunts can go in three different directions, toward the pitcher, toward the third basemen or 1st basemen, they need to be there to cover the ball, when getting the ball they can go to the 2nd basemen who is covering 1st for the out or to the ss who is covering 3rd. Most of time is this situation the throw will go toward first.

In some situations you should have the third baseman stay back, especially if the pitcher is a lefty (lefties push off toward the thirdbase dugout) so that the pitcher can field the ball and the thirdbaseman can cover third. This makes especially good scence when the players are ages 10 to 15 where the Pitcher is usually the best athelete on the field, and therefore the best choice to field and throw.

If you have the first and thirdbase fielders charging, shortstop covering third and 2ndbaseman covering first, make sure someone covers 2nd base (maybe the centerfielder) or if the runner on 1st is not put out he/she may keep running.

Typically, you will have a couple different plays that you practice and then apply based on the circumstances. A wheel play has the shortstop get a running start towards 3rd, and the 1st and 3rd basemen crash before the pitch is thrown. This is when you know the batter is bunting, and you are trying to get the lead runner at 3rd.

A more conservative play has the pitcher cover the 3rd base side as well as right back to the mound, and 1st baseman charges. In this scenario, 3rd baseman only comes on a hard hit bunt (that gets past the pitcher), but otherwise stays back to cover the base. Typically, this will get the out at 1st unless it is a hard, fast bunt back to the pitcher.