The fact that she was even able to be born at all is the loophole. But her keeping her magic after turning is/would be a plot hole. Because she still has too die to officially become a vampire just like every other vampire in existence. And just like every other witch in existence, nature can still take away her magic the exact same way. So her having all three at the same time isn't a loophole, it's the writers not caring. She shouldnt even be allowed to use magic after triggering her werewolf curse (unless the magic she's using is actually the Hollow's), the first werewolves were witches before they were cursed yet none of them were able to practice magic afterwards abd none of their descendants were witches, proving you cannot be both active at the same time.

However, I am still going to go with my head canon that she was not an accidental/unforeseen loophole, and that her birth was actually deliberate because there is another being out there that is way to powerful (older and more powerful than every other we've seen since the beginning of TVD; Cade, Silas, Qetsiyah, Bonnie, Markos, Dahlia, Esther, Hollow, ect) and Nature feels the only to stop it is a creature that is a combination of all three main species; witch vampire werewolf. It'll still technically be going against already established mythology, but at least thered be a valid reason for it, aside from the current "because i say so" from the show runners. 

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