A Triple Battle is a battling system that first appeared in the Generation V games Pokemon Black and White. Similar to Double Battles, trainers use three Pokémon at a time. Though it first appeared in-game in Pokémon Black and White, Triple Battles were mentioned in the anime during the Battle Frontier arc.
In Triple Battles, the Pokémon on the far right cannot attack the Pokémon on the far left and vice versa, but moves that are long range like Dragon Pulse and special moves like Acrobatics evade this rule. Pokémon can also be repositioned on the battle field by pressing the arrow button on the battle menu, but doing this costs the switched Pokémon's turn.
Though not yet seen in the anime, they were mentioned when Ash traveled through Kanto to battle various Frontier Brains. When Ash fought Arena Tycoon Greta, her battle rules involved spinning a wheel to decide if the battle would be Single, Double, or a Triple Battle, but Ash got a Double Battle.