After defeating Blueno with Gear 2, Luffy faces a massive steel door blocking his path to the Tower of Justice. He activates Gear 3 to blast through it.
He unleashes the Gomu Gomu no Gigant Pistol , a punch so massive it rivals the size of a Giant's fist, nearly knocking Lucci unconscious and shattering a Buster Call battleship in the process. Evolution of the Form which episode luffy use gear 3