Greater Multiweapon Fighting( Savage Species, p. 35)
A creature with three or more arms can fight with a weapon in each hand. The creature can make up to three attacks per round with each extra weapon.
You may make up to three attacks with each offhand weapon that you wield, albeit with a -10 penalty on the third attack with each such weapon.
This feat replaces the Greater Two-Weapon Fighting feat (originally presented in Masters of the Wild) for creatures with more than two arms.