You'd think with three of the most idolized characters in fanboyism this would have at least one reply....
In any case, here are some feats of theirs to help you decide:
Neo Exdeath: The embodiment of the Void, an extremel powerful force of nohingness. Capable of erasing towns from existence with ease thanks to the Void.
Trance Kuja: Kuja in a powered form. Has powerful magic spells and nuked the surface of a planet in a very short amount of time.
AC Sephiroth: Even using no effort he can move faster than the eye can see, teleport, slice skyscrapers to pieces in seconds, fly, and con trol his own lifestream. He has powerful magics like Supernova and Heartless Angel, strong telekinetic powers, and incredible strength and speed.
God Kefka: The literal god of magic. Has all of the power of the warring triad, can incinerate cities anywhere in the world with a single use of Light of Judgment, and has powerful spells like Forsaken.
True enough, he has the potential to erase existence, but I don;t believe he can do it instantaneously or he would have done it in FF V against Bartz and company. The question here is can they take him out via speedblitz or something before he Voids there asses.
As for this matchup hmm...I don't think how any of them can avoid getting erased...however how is ExDeath's durability, because these 3 combined pack quite a punch.
Exdeath has regenerated from a splinter, but he was defeated for good in his final form by people with no formal combat training. They had the powers of the four elemental crystals, but we're not sure how much this amped them and in what aspects.
My money says that individually they're toast, but as a team they could walk away with 4 or 5/10 if they hit him very fast with a simultaneous Ultima, Supernova and Light of Judgment.