Appearances in the anime
After successfully cloning Mew and creating Mewtwo, one of the scientists exclaims that they should "get started on Mewthree".
Appearances in the manga
Pokémon Pocket Monsters Manga
In the Pocket Monsters Manga, Red's Clefairy becomes Mewthree after merging it's own DNA with that of Mewtwo's. Clefairy/Mewthree proceeds to battle and defeat Mewtwo before being reverted to normal.