Rule of Three

The rule of three is a writing principle that suggests that a trio of events or characters is more humorous, satisfying, or effective than other numbers. The audience of this form of text is also thereby more likely to remember the information conveyed because having three entities combines both brevity and rhythm with having the smallest amount of information to create a pattern.

This applies not just to writing but to many others in life -- pick any three colors and use them together, any three musical notes and they form a pleasing chord, any three simple flavors and most people will love them.