Is There a Mewthree and where can you find it

No. There is only a mew and mewtwo. You cant find it as it hasn't been created.
Mewtwo is a cloned version of mew gone bad. Example is Pokemon movie with mewtwo in it they try to clone mew by putting its hair into a cloning machine but failure results of this as they do things to the new mew which they named mewtwo which turned him evil and against humans.

If you have an action replay a code for a Pokemon named Mewthree for pearl is:
94000130 FCFF0000
B21C4D28 00000000
B0000004 00000000
E0000304 000000EC
F8D0F8D1 4AF70000
B48FC81D BA78A427
9EB0E6A3 7A0EB95B
E920512E 3F671249
447E351C 92B70D74
EC5D6C18 512C8B2C
E144DE29 A604CB74
4E990503 CD3A55FA
ACA85D57 B1949C5B
95F1C326 2D5BB449
D014D8A2 291F3939
6ABB8717 DC05BF01
0F734551 6E9C49AA
F15364D5 6543A7F3





UPDATED by lorddaro