Once you notice the rule of three you cannot stop seeing it, and the reason it is everywhere is mechanical rather than mystical.

The mechanism

One event is an incident. Two might be coincidence. Three establishes a pattern — and the third is where the pattern can be confirmed or broken, which is where all the meaning lives. The eldest brother fails, the second fails the same way, and the youngest succeeds by doing something different. The audience has been taught the rule by repetition and then shown the exception.

Why oral tradition in particular

• A listener cannot turn back a page, so repetition is how information is retained.
• A teller working from memory is helped enormously by a repeating frame with one variable.
• The repetition builds anticipation. By the third instance the audience knows the shape and is waiting to see how it will differ, which is a more active form of attention than surprise.

Written storytelling inherited the structure and often uses it out of habit. In an oral tradition it was load-bearing — remove it and neither the teller nor the listener can hold the story.