Title: Apprentice Triad

Main characters: Twilight Sparkle, Sunset Shimmer, Trixie, Princess Celestia, Princess Luna, Tirek.

Genre: Alternate Universe, Dark, Adventure, Slice of Life.

Premise:
Princess Celestia discovered a Unicorn with great magical potential named Sunset Shimmer. She took her under her wing to teach how to control her power better. However, little did she know the she wasn’t the only Unicorn that held great potential…

During the entrance exam of a young Twilight Sparkle, a distant explosion causes her to lose control of her magic and create a magic burst that hatches the dragon egg. Sunset Shimmer arrives and tries to calm the young unicorn down, however she gets knocked back by the blast. Much to her shock however, a young blue unicorn named Trixie uses her magic to contain the burst long enough to snap Twilight out of her trance. While they're distracted, an unknown figure retrieves the newly hatched baby dragon...

Celestia arrives and applauds both unicorns for doing a good job containing and stopping the magic burst, also stating that Twilight has very high potential if she learns to control her magic. As a result both Twilight and Trixie end up becoming Celestia's newest student's, studying under both her and Sunset Shimmer.

However many years later, the trio comes across an ancient prophecy that foretells the coming of a demon from Tartarus...and the only thing that can defeat him is the Elements of Virtue…

I would definitely read that.