the third attack will do extra damage

I want to do a especial "power" to an weapon. I want to do something like, when the weapon first hit hits an mob, does nothing; when the weapon second hit hits an mob, does nothing; when the weapon third  hit hits an mob, does extra damage, and start the cycle again.

if someone help me how to do that, I'll me grateful （￣︶￣）↗　.