You can't unless You have pokesaved a mew
ye you can
Catch trainer Pokemon
Mewthree cannot be caught in Pokemon Diamond, even with an Action Replay, because it does not exist in the game's data.
if its action replay than look up action replay codes on google
No you cannot.
You can't.
trade or migrate or action replay
not without an action replay and a Pokemon modifier code.
get an action replay, get the Pokemon modifier cheat for it, then catch all the Pokemon.
No, there is no legitimate Action Replay code for Mewthree for Pokemon Diamond.
you use the ds action replay or you migrate it
You can't unless you have an Action Replay Ds