Actually, There is a MewThree, it can be found in the Chaos Realm, or Glitch City (Chaos Realm sounds cooler). In Chaos Realm, you can find the 9th Gym, and battle noneother than Missingno for the Mist Badge.
(Chaos Realm Gym is speculation, but due to the many glitches discovered in this game, it can't be ruled as impposible)
Get Your Game On!