No, it's not. Fairy Garden uses the Pentagon Rule, that only allows Pokémon caught or bred in Pokémon X/Y. And as far as I know, you can't catch an Arceus in X/Y. It needs to be banned.
Also, despite being 3v3 and you pick 3 Pokémon in Showdown for Fairy Faceoff, it is still a 6v6 match...