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The 3-5-2 formation is used by teams whose players know to work together. Just a bit more offensive than the 4-4-2. The defenders need to communicate well, and the midfield has to be included into defending.

Here are some details about the positions:

Sweeper (SW) In this formation, the sweeper needs absolute control over the defenders. The communication between the defenders has to be almost perfect, since they are more likely to be out numbered than the 4-4-2 defenders.
Stopper (ST) Marks a forward, and also organizes the defense.
Left/Right Defenders (LD/RD) Communication is the key to a stabile 3 men defense. Should be less active in offensive plays unless the midfielders can fill in.
Left/Right Centermidfielders (LC/RC) Both center midfielders play a defensive part in this formation. They cover for the center midfielder who leads the offensive plays, and at least one of them functions as a defender when the ball is in their own half of the pitch.
Left/right Midfielders (LM/RM) Need to call for the ball more than usual, since a team with a central midfielder tends to drive the ball up the middle rather than the outsides.
Centermidfielder (CM) This player needs good ball handling and passing skills, since the CM has the control over the offensive plays, distribution of the ball in the opponent's half, and standard situations.
Forwards (F1/F2) One of these should occasionally drop back to support the center midfielder.