D20:Greater Two-Weapon Fighting Feat

Dex 19, Improved Two-Weapon Fighting, Two-Weapon Fighting, base attack bonus +11.

You get a third attack with your off-hand weapon, albeit at a ?10 penalty.

A fighter may select Greater Two-Weapon Fighting as one of his fighter bonus feats.
An 11th-level ranger who has chosen the two-weapon combat style is treated as having Greater Two-Weapon Fighting, even if he does not have the prerequisites for it, but only when he is wearing light or no armor.

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System Reference Document -> Feats