"Oz and the Three Witches" is a short story written by Hugh Pendexter III. First published in booklet form in 1977, it was reprinted in the final issue of Oz-story Magazine in 2000 (with original artwork by Patricia Ambrose).
Pendexter's story fills a gap in the general Oz mythos. It is set at a specific point in Oz chronology, just after the conclusion of Baum's fourth Oz book, Dorothy and the Wizard in Oz. Princess Ozma has invited the Wizard to remain in Oz and become her student. Glinda, however, is less immediately welcoming; she comes to the Emerald City and gently but persistently interrogates the Wizard with her Truth Pearl, on his actions and motives during his first stay in Oz — as to why he delivered the infant Ozma to Mombi, and how he survived against the Wicked Witches of the East and West with no magic of his own. (In their climactic battle, the Wizard tricked the two witches into fighting against each other.) Only when he satisfies her does Glinda conclude that the Wizard is indeed a good man, and a fit candidate for an education in magic.
One interesting aspect of the tale is its use of the Truth Pearl. This magical talisman was introduced by Baum in the penultimate chapter of The Marvelous Land of Oz. There, Glinda has "an immense white pearl, attached to a slender chain" that functions as a lie detector: lies told in its presence cause the Pearl to discolor to black. Unlike other elements of Ozian magic, like the Great Book of Records, the Magic Belt, and the the Forbidden Fountain, Baum never mentioned the Truth Pearl again in his later Oz works; and his followers and imitators have generally continued that neglect.