Good megas? for 3rd gym?

Is mega flygon good or mega delta venusaur better?

Mega Delta Venusaur. Half of the Gym Leader’s team is Dragon-type. Therefore, it’ll have a pretty big advantage, especially considering Hubris special attack boosts and Moonblasts if you’re at level 50. Just make sure that you have something to deal with his Skarmory and you should be fine.

Mega Flygon would work pretty well too, but you’d need the Move Relearner to gain the most out of his ability and there are 3 Pokemon on his team that can hit you super effectively. That’s not to say it’s bad; it’s probably one of the most broken megas in the game and can definitely do well in this battle, but in my opinion, Mega Delta Venusaur will serve you better specifically for the third gym.