The 3-4-3 formation is perhaps the most offensively concerned start up formation. It's used against teams that are expected to play with a strong defense, or in cases when several goals are needed. The positions are basically the same as in other startups. The only difference might be that the outside forwards need to make runs to the corners, since the outside midfielders are rather busy with defensive jobs.
Here are some details about the positions:
Sweeper (SW) The sweeper should be a
leader on the field. Controls the defense, always staying behind the fullbacks.
Left/Right Backs (LB/RB) Cover the opponent's forwards..
Left/Right Defenders (LD/RD) They mark (or cover) the other teams forwards. With more advanced teams these players are used to carry the ball up the outside with the midfielders, allowing more effective crossing.
Left/Right Midfielders(LM/RM) In charge for the crosses in the offense, and usually carry the ball up the field, since they have more room than the center midfielders.
Left/Right Center Midfielders (LC/RC) Distribute the ball among the offenders and try to stop or slow down opponents plays before they get to the defensive line.
Forwards (F1/F2/F3) Runs to the corners are the primary weapons of forwards. The center forward and the opposite forward should break inside to receive the cross.