The rule of three is a writing principle which suggests that a trio of entities such as examples, events, or characters is more satisfying, effective, or humorous than other numbers, hence also more memorable, because it combines both brevity and rhythm with the smallest amount of information needed to create a pattern.

Slogans, film titles, and a variety of other things have been structured in threes, a tradition that grew out of oral storytelling and continues in narrative fiction. Examples include "The Three Little Pigs", "Goldilocks and the Three Bears", and The Three Musketeers. Similarly, adjectives are often grouped in threes to emphasize an idea.

The rule of three can refer to a collection of three words, phrases, sentences, lines, paragraphs/stanzas, chapters/sections of writing and even whole books. The three elements together are known as a triad. The technique is used not just in prose, but also in poetry, oral storytelling, films, and advertising.

A tricolon is a more specific use of the rule of three where three words or phrases are equal in length and grammatical form.

A hendiatris is a figure of speech where three successive words are used to express a single central idea. As a slogan or motto, this is known as a tripartite motto.

Many advertising campaigns and public information slogans use the technique to create a catchy, memorable way of displaying information. In marketing theory, American advertising and sales pioneer E. St. Elmo Lewis laid out his three chief copywriting principles, which he felt were crucial for effective advertising:

The mission of an advertisement is to attract a reader so that he will look at the advertisement and start to read it; then to interest him, so that he will continue to read it; then to convince him, so that when he has read it, he will believe it. If an advertisement contains these three qualities of success, it is a successful advertisement.

Some examples include: