Godzilla 1954=GvM Millennium Godzilla
Kiryu=I don't know how this fits....
Mecha-KIng Ghidorah=Heisei era version. It's also controlled by the Vortaak, just like MechaGodzilla.
KIng Ghidorah=Version from the Atari timeline that is unexplored.
That's a couple of neat theories I came up with to explain why in that timeline there's 3 Godzillas and the 1954 one should
Spoiler: