Mega Mawile is way better than Mega Gardevoir/Gallade most of the time. Huge Power literally gives it the highest attack stat in the game and it's typing is way better. Mega Mawile has 9 resistance (one of them being 4x), 2 immunities and only 2 weakness. Meanwhile Mega Gardevoir has 2 resistance (one of them being 4x), 1 immunity and 3 weakness and Mega Gallade has 2 resistance, 3 weakness, and no immunity.