I think I figured out why Charizard's max move does what it does and what the other two will do as a result.

G-Max Wildfire has an effect almost identical to the Fire Pledge's sea of fire. Because of this I'm guessing that G-Max Venusaur and Blastoise's max moves will do the same, but for their respective Pledge moves. So, G-Max Venusaur would create a swamp on the enemy field that quarters enemy speed for four turns, and G-Max Blastoise would create a Rainbow on the ally side that'd double the chances of secondary effects happening. I can't believe people have glossed over this possibility.