Grendizer, Getter Robo G, Great Mazinger: Kessen! Daikaijuu!

On a solitary island in the middle of a vast unknown ocean, Dr. Hell, a deranged genius, plans the final stage of his plan for world domination: the activation of his Mecha Beast Army. The only man aware of Dr. Hell's plan is Dr. Kabuto, a pioneer in new energy sources. In a desperate attempt to stop Dr. Hell before his death, Kabuto uses his research and resources to construct a weapon powerful enough to match that of the Mecha Beast Army, which he entrusts to his grandson, Kouji Kabuto. That weapon is the Super Robot Mazinger Z!

Kappei is your normal run of the mill delinquent lad, riding motorcycles and trying to gain leadership of a gang. When fighting with a rival, they are both attacked by a strange creature! Fleeing to his Grandfather's nearby sea rig, Kappei is knocked into the water and is rescued by a giant spaceship piloted by his grandparents. Before he can figure out what is going on, he finds himself in the mighty Zam-bird, fighting against the monster that attacked him - a mecha-boost of the planet-destroying Gaizok! Kappei isn't the only one in his family who ends up with a giant robot, though; his cousin Uchuta pilots the Zambull and Keiko pilots the Zambase. Together, the three of them can transform into the Invincible Superman Zambot 3!