Someone once pointed out to me why folktales often have events that happen in threes--

Three is the smallest number in which you can establish a pattern and then break the pattern.

The first two have something in common; the third thing is different from the first two in an important way.

In the Three Little Pigs story, the first two fail to keep the wolf out of their houses; the third one succeeds.

Sometimes it takes two failed attempts for the hero to figure out what he is doing wrong (and for the storyteller to make it clear to the listener); then he applies his new knowledge and succeeds.