It's one weird trick (or one trick with three parts): the rule of three. In Latin, it’s mne trium perfectum (every set of three is complete).
When you’re just starting on your project, or if you are stuck, think about how you could divide your work into three parts.
Think about it: Larry, Curly, and Moe. Red, White, and Blue. The Three Musketeers. The Ghosts of Christmas Past, Present, and Future. The three little pigs. And doesn’t “Crosby, Stills, and Nash” sound more natural than “Crosby, Stills, Nash, and Young?”
Not to mention the three-act structure used in screenplays, and the three-item bullet lists favored by click-bait web content writers.
Not convinced? A web search reveals dozens of articles on the Rule of Three. How to use it to write backstory, for example. Or how to structure a joke. Or a speech. (See what I did there?)
We are hardwired to notice patterns, and three of something is the smallest pattern. The rule of three helps us give a structure to our writing so that our readers or listeners will grasp our message more intuitively.
--Ann Kellett, Ph.D.
Ann Kellett Editing