This formation is quite an attacking one.
The 4-3-3 system - that is four defenders, three midfielders and three attackers - allows you to have three attackers while still having a strong midfield.
The three midfielders will normally play quite closely together to give protection to the defence and so they cannot be passed around by the opposition.
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The three strikers will split across the pitch to provide width to the attack.
Usually you would have a central attacker with one each side of him.
Barcelona play with this system with Samuel Eto'o, Ronaldinho and Ludovic Giuly.
Very few teams actually start with this line-up as you need to have specialist attackers.
But if a manager needs a goal late on in a game then he may opt to move to three up front.