Densetsu Kyojin Ideon

From the continuing reclamation of the ruins of the planet Solo, mysterious mechas are excavated. But suddenly, an alien called Buff Clan attacks seeking the limitless energy “Ide.” The earthlings are backed into a corner, when the three mechas piloted by the earth boy Yuki Cosmo and others fuse together. Becoming the giant robot Ideon, they use their incredible power to fight back against Buff Clan.
(Source: Sunrise)