Why does O. Henry use the Number 3 as a significant figure in his story "The Gift of the Magi"?

The number three is also a recurring element in a fairy tale. (Incidentally, the numbers 5, 7, and 9 come up sporadically, too.) There are often three sisters, three wishes, three challenges, etc., in which two of these elements are often foils (elements to show contrast) for the third.  Take, for example, the wicked step-sisters of Cinderella. Their ugliness, selfishness and outright meanness contrast with her gentle and noble nature.

In storytelling, this is just enough content to get the story going, develop the suspense until the crisis point, then make a resolution and denouement. This conveniently corresponds to a bedtime story, which lasts between five and ten minutes. However, I think the main point goes along with the first, since along with the element of three is the famous happily-ever-after ending, which is also a signal trait of the fairy tale. Such, at least, is the supposed destiny of Jim and Della.