Title: The Triad Of Magic

Main characters: Twilight Sparkle, Sunset Shimmer, Trixie.

Genre: Alternate Universe, Crossover, Adventure.

Crossover With:
The Lunaverse
The Shimmerverse

Canonical Placement:
Cannonverse: Directly after "Equestria Girls: Rainbow Rocks"
Lunaverse: After "Crisis on Two Equestrias".
Shimmerverse: After "A Different Sunset"

Premise:
The Element weilders are called to Canterlot one day by Celestia, according to her the Element of Magic has started acting up. Upon casting a spell on it, the Element ends up leadinh her towards an interesting mirror. Suddenly the mirror reacts to the Element's presence and sucks Sunset in and closes before anyone else can enter...

In another version of Equestria, Trixie and her friends follow a strange pulse that seems to be calling to her Magic Element, the pulse leads her to an interesting mirror in the Castle Ruin's of Everfree Forest. However when she attempts to lift it with her magic, it reacts to her Element and sucks her in as well...

The two awaken and find themselves in an alternate version of a Equestria where they are confronted by Twilight Sparkle herself. However before they can question what's going on, their Elements detach themselves from their crowns before flying off towards the Everfree Forest. They quickly follow the Elements and end up the Tree of Harmony itself, where this universe Element of Magic merges with the other two. Upon doing so, they transform into an Alicoron and brings forth a horrifying truth:

Antithesis has returned, and she is NOT happy...