All Final Stage Pokemon scale to Charizard and are having their Profiles updated to the latest revision, so you can't say Zard stomps because of tier. For Non-megas, Venusaur is in a tight spot since it's pretty much stomped by Zard's Dual Fire and Flying typing and Blastoise has the ability to fight him off thanks to the access to moves like Ice Beam so he can't easily stomp Blastoise in return. So it's Blastoise vs Charizard as mentioned before by others and the results are self evident, Zard can't really defeat Blastoise simply because the best he can do is to use Solar Beam and try to get an advantage with Flight, which are both easily handled by Blastoise since Zard requires charging to use Solar Beam, letting him exposed to get hit by a point blank Hydro Cannon, and Blastoise has two ************* cannons capable of launching powerful water based attack several meters, so Zard can't really just show his wings and fly way expecting to win. Round 1 Blastoise wins.
Now for the Megas. Venusaur is much more comfortable now, Thick Fat compensates the Ice and Fire weaknesses, so he can't be simply destroyed by Zard's fire moves and Blastoise gets a hard time trying to fighting him since he now can't count on Ice type moves to fight back. Charizard also now can easily keep up with Blastoise, Zard X loses water weakness and has acess to powerful Dragon type attacks that hit Blastoise neutrally, while Tough Claws makes his main Dragon move, Dragon Claw and other physical moves 30% stronger than before. Zard can also go into his Y form, which summons intense sunlight that boosts Fire type attacks by 1.5x and halves the damage of water attacks. This also gives him Solar Beam without having to charge so he can easily spam this super effective move against Blastoise. If anyone is going to get defeated first, it's Mega Blastoise, though this would be much closer than the round before since Blastoise still gets much more powerful and can cause major damage against Charizard and keep up to an extent with Venusaur, but this match-up is just really not favorable to him. Now it's Mega Charizard vs Mega Venusaur. Mega Charizard gets this since Mega Venusaur would struggle against both Megas. Mega Zard X may have Fire moves nerfed a bit by thick fat, but they're still effective, specially the physical moves like Fire Punch and Flare Blitz, and access to powerful Dragon moves if this isn't enough, while Venusaur wouldn't be able to hurt him much with grass moves. His best chances of winning are with Poison moves, but with this he would struggle getting good hits because of Zard's flight, and moves like Sludge Bomb are clearly not as effective ranged attacks as Blastoise's cannons. Zard Y can win similarly, Venusaur can struggle to get a hit and Zard Y can hit Venusaur hard with sun to compensate for the Thick Fat ability and Flying attacks like Air Slash that hit super effectively while still maintaining range. For Round 2, it's Mega Charizard.