idea

A trio of something is more pleasing, humorous, satisfying than other combinations.

It's applicable to story-structure:

simple three: Three somethings happen, the last one is the conclusion
ascending three: Win bronze, silver, then gold
contrasting three: the third one contrasts with the rest (last stand, three little pigs)
dialectal three: the first and second are wrong in different ways, the third is just right (goldilocks)

references