All of the three titles you suggest in a way claim to be more than one story. "The Untold Stories" and "The Untold Chapters" in particular seem to claim to fill up all or most of the remaining LBT plot-holes created by earlier movies while giving no information whatsoever on the content of the story. Unless the fanfiction can live up to that claim I would pick a different title.
Stories from the Mysterious Beyond gives some suggestion on the whereabouts of the fanfiction and by just leaving out the word "the" it does not go as far in claiming something all-including, but it also suggests several stories rather than one.
Perhaps it would be a good idea to think of a title referring to the one story you mean to write. All of the titles you brought up so far sound a bit like a head title for a whole series of stories.
