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Could this be one of the many versions of Getter Robo? The configuration you're describing doesn't remind me of any of the Getter machines I'm familiar with, but the idea of three piloted machines combining like that sounds like it could be from one of them.
Edited by Durazno
GoShogun had a somewhat similar setup (three vehicles combining into a mecha, then breaking up and entering a bigger mecha). It's certainly known in Europe.
It was Go Shogun or as I knew it Macron 1
Edited by dutchguy1986
Happy to help.
I'm looking for a mecha anime, it aired during in the Netherlands but I'm not sure how old it really is. Here is what I remember there were three pilots with individual mecha. Naturally they combined in the regular way where each robot transforms either the arms legs or torso and head but they also had something I think was unique.
There was a fourth mecha separate from the three individual ones and they basically flew into from an opening in the back and transformed into their limb modes before it closed up again. Both combined forms typically required all three pilots or at least two of them to control.
At one point the pilots of the mechs that formed the arms and legs were in trouble without mechs and were saved by the four mech combined form assuming the remaining piot did all the work. In truth a support character, who I think was being trained as an emergency replacement for any of the three and who had messed with the settings of the mechs to make it easier for him, was acting as the second pilot.
Edited by dutchguy1986