Setting: Mt. Fuji.
GMK Godzilla faces off against the GFW versions of King Caesar, Angilas, and Rodan (all scaled down to 60 meters).
Would GMK G be able to defeat the trio, or would they get their first victory?
I think the trio has a decent chance of winning. GMK G is much slower than the Godzilla they faced, and the three of them will be able to run circles around him. I doubt GMK G would be able to handle them attacking all at once, and I think Rodan's diving peck attack has a good chance of breaking a hole through G's skin. Angilas' spinning ball attack would most definitely poke some holes.
Not to say that GMK G will be an easy defeat. His ray, while not as strong as GFW G's, is still nothing to sneeze at. It will hurt them, but I think they'll be able to withstand it better than the Guardian Monsters. If he manages to grab one of them, he'll be able to toss them around and do some damage, and his tail whip will send any of them for a loop.
Ultimately, though, I think the fight hinges on how badly GMK G's ray will damage the trio. If it doesn't have the same lethal power as it did against the Guardians, I think Caesar, Rodan, and Angilas will win.