My Little Pony: Friendship is Magic - The Sunset and Starlight Show

Fighting over who gets to be Twilight's Arch-Rival…

I know it's not possible now since all 3 of them are reformed, but I would've given anything to see an episode where the 3 of them try to simultaneously take revenge on Twilight. However, the twist is that they're coming at her simultaneously and each without knowledge of the other two, so they keep unintentionally interfering with the others' attacks. The episode would pretty much end with the three of them fighting each other over these interferences, with Twilight never realizing that somepony was ever trying to attack her.

I dunno, maybe it'd be too comedic for a three-villain appearance, but I still would've liked it.