Starvengers: The Movie (1976)

Soon after the events of the first Getta Robo series, Earth finds itself under attack by the Hyakki (Pandemonium) Empire and its fleet of horned battle robots. In response to the threat, the Getta labs re-fit their chief weapon: three advanced jets, capable of merging into three distinct robots. These are Getta Dragun (Star Dragon) the most powerful and oft-used formation; Getta Rygar (Star Arrow) designed for speed and aerial combat; and Getta Poseidon (Star Poseidon) built for strength and aquatic battle.

Unfortunately the movie Starvengers: The Movie is not yet available on HBO Max.