I GUESS it's technically four because of Robogodzilla (a Mechagodzilla-like monster seen at the end of Singular Point and was built using a Godzilla skeleton)
IF I HAD A NICKEL FOR EVERY TIME MECHAGODZILLA WAS BUILT USING PARTS FROM OTHER KAIJU; I'D HAVE THREE NICKELS, WHICH ISN'T A LOT, BUT IT'S WEIRD THAT IT HAPPENED THREE TIMES