Aristotle made the observation that people find it easiest to remember three things. The rule of three started with his writing, “The Rhetoric;” In fact, in many of the areas where the rule of three is practiced (and there are a lot of them), it is referred to as rhetoric.
In computer programming, the rule of three is a “rule of thumb” for class method definitions as well as code refactoring. It is a computation method used in mathematics and in statistics it is used for calculating a confidence limit when no events have been observed. In aviation, it is a rule of descent and in economics a rule of thumb regarding major competitors in a free market.