The third drink is the best, allowing us to dance freely and shoot gorgeous pool. (The fourth does us in.) Three brothers Karamazov, three brothers Marx, the Three Musketeers, three little pigs, three blind mice, three-course meals.

Three of something seems complete. It’s a number that holds an almost magical power over the human mind. And when we encounter something too complex or messy to understand, our first instinct is to split it into three parts. Young writers tend to group their adjectives in threes—a habit that is sleepy, predictable and comforting. Threes are everywhere in Shakespeare (“Tomorrow and tomorrow and tomorrow…”), and form one of the most common rhythmic devices in poetry. Riddles, too, are best in threes. Think of Oedipus and his Sphinx. When it comes to...