there are three similar rhythms and then a final action… The three are always clear units: 1, 2, 3, with a certain similar repetition, which is why the fourth is so often ignored, for the fourth is not just another additional number unit; it is not another thing of the same kind, but something completely different.
Von Franz establishes that three functions as a rhythmic, repetitive structure in fairy tales, distinct in kind from the transformative fourth that follows it.
, The Interpretation of Fairy Tales, 1970thesis