Here is an overview of the game, via its store pages:
Start a new adventure in the Pokemonworld! Explore the all-new island of Pasio! There’s a tournament of three-on-three battles being held on Pasio called the Pokemon Masters League. Aim to become its champion as you battle with famous Trainers and make a dream team of friends on the way! Form a team with three Trainers and their Pokémon to target your opponents' weaknesses and deal big damage! Enjoy co-op play with friends all around the world! In co-op play, three players team up for fun, exciting battles! Work together to emerge victorious!