Grendizer, Getter Robo G, Great Mazinger: Decisive Battle! The Great Sea Monster (1976)

A giant sea monster known as Dragonsaurus has emerged from out of nowhere, terrorizing the depths of the oceans. Great Mazinger, Grendizer and the Getter Robo G team join forces to combat the new threat. But their task becomes more complicated when Dragonsaurus swallows up Boss Borot and makes its way towards Tokyo. Source: ANN

Unfortunately the movie Grendizer, Getter Robo G, Great Mazinger: Decisive Battle! The Great Sea Monster is not yet available on HBO Max.