Improved Multiweapon Fighting( Savage Species, p. 36)
You are expert at fighting with a weapon in each of your three or more hands. You can make up to two attacks per round with each off-hand weapon.
Multiweapon Fighting (MM) , DEX 15, Base attack bonus +9, Multidexterity, three arms,
In addition to the single extra attack you get each round with each off-hand weapon from Multiweapon Fighting, you get a second extra attack with each such weapon, albeit at a -5 penalty.
This feat replaces the Improved Two-Weapon Fighting feat for creatures with more than two arms.
With only Multiweapon Fighting, you get only a single attack with each off-hand weapon.