Goldilocks and the Three Bears. The Three Musketeers. The Three Stooges.
In storytelling and comedy, the number three is magic. But why? Why not two? Why not four?
The Pattern: Setup, Setup, Twist
The Rule of Three works because it is the minimum number required to establish a pattern and then break it. 1. Item 1: Establish the pattern. (The Englishman orders a beer). 2. Item 2: Reinforce the pattern. (The Scotsman orders a whiskey). 3. Item 3: Break the pattern. (The Irishman orders... something unexpected).
If you only have two items, you haven't established a strong enough pattern to subvert. If you have four, the audience gets bored waiting for the punchline.
Rhythmic Comedy
Comedy is music. The "One, Two, Three" structure provides a satisfying rhythm. * Setup (Da) * Reinforce (Da) * Punchline (DUM!)
Examples in Action
The Dick Van Dyke Show: "Can I get you anything? Cup of coffee? Doughnut? Toupee?"
Jon Stewart: "I have three rules for this show. 1. No whining. 2. No crying. 3. No whining."
Classic Joke Structure: "I want to die peacefully in my sleep, like my grandfather. Not screaming and terrified like the passengers in his car." (Note the structure: 1. Peaceful desire. 2. Grandfather comparison. 3. Horrifying twist.)
Breaking the Rule
Advanced comedians sometimes break the rule of three to mess with the audience. They might do a "Rule of Seven," repeating a list until it becomes boring... and then repeating it so many times it becomes funny again (The "Sideshow Bob Rake Effect").
But for 99% of jokes, three is the magic number.