I'm alright with Mega-Blastoise, although I'm not sure about those two arm cannons. When you already have a huge cannon on your back that reaches forward over your head, two little super-soakers seem kind of pointless.
Mega-Venusaur... did they even change you? A few face markings, an extra flower, some slight plant modifications that aren't even noticeable unless you have a before and after picture available.
For Mega-Charizard, I dislike those little wrist-wings or whatever they are. Other than that, it's alright.