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The 4-3-3 formation is not completely defined since it is not used very often. The roles of the three forwards, however, are  very clear: two should be making runs to the outsides and one either dropping back or waiting for the cross. The midfielders adapt their runs and running paths to those of the forwards. The defense can't count on much support from the midfield.

Here are some details about the positions:

Left/Right Backs (LB/RB) The only real defenders. These players have to be dependable and willing to give up any kind of offensive play.
Left/Right Defenders (LD/RD) They mark (or cover) the other teams forwards. These players can be used to carry the ball up the outside with the midfielders, allowing more effective crossing. Can also be used to provide more defense.
Center Midfielders (LC/CM/RC) Distribute the ball among the offenders and try to stop or slow down the other teams plays before they get to the defensive line. Also, the offense depends on occasional crosses and long balls down the lines, since the 3 midfielders cannot work the ball forward.
Forwards (F1/F2/F3) Runs to the corners to feed the offense, and dropping back is usually the only way of getting the ball, due to just 3 midfielders.