Mewthree

Mewthree is the fan depiction of a second clone from Mew, and the clone after Mewtwo.

Origin
Like many other Pokémon, Mewtwo was thought to have another evolution. This led to the creation of Mewthree. It is interesting to note, thought, that Mewtwo was a legendary Pokémon, which usually did not have any further evolutions even in the form of PokéGods. Mewtwo's status as the strongest legally obtainable Pokémon in the games may have been the reason it did receive this evolution.

Mewthree was named as such because Mewtwo was clearly the second version of Mew and much stronger, leading fans to believe that there is a third version that is even more powerful. Many believed that the appearance of Mewtwo in armor from Pokémon - The First Movie - Mewtwo Strikes Back was Mewthree because it is stated to be more powerful than Mewtwo.

Mewthree is not further confirmed in the Pokemon franchise, though in the Pokemon Pocket Monsters manga, Red's Clefairy absorbs mewtwo's DNA and transforms into it, dubbing himself as Mewthree.