Tag Alias: mewthree -> mega_mewtwo

Reason: mewthree was merely the placeholder/common fan name for mega mewtwo before we knew what it was called. Now that we know it is called mega mewtwo and the appropriate "mega_mewtwo" tag was created, the tag "mewthree" no longer serves a purpose. Let's face it, there never will be a mewthree for as long as we or this site lives.

+1 to this. Mega_mewtwo has been confirmed by official sites as well as other sites like serebii

I've already made this suggestion, but with typo, in forum #79470