SuperMarioGlitchy3 or more commonly abbreviated as SMG3 is the evil version of SuperMarioGlitchy4. In the Battle of the Retarded Villains series, he competed with EM64 to win the battle of who is the best villain.
He first appeared in Super Mario 64 Bloopers: SMG4 VS SMG3, where he copied SMG4's "War of the Fat Italians" video.
In the SMG4 series, it is revealed that SMG3 is a Super Meme Guardian. But it should be noted that the SMG4 series is NOT canon to the OnyxKing series as revealed in "OnyxKing vs The Pokemon Championships Part 1".
It is currently unknown if he will appear again. As SMG4 was written out, but SMG3 was implied at the end of "Mario EndGame Pt.2" to still be a recurring antagonist.