Sometimes called trebling, the Rule of Three is a pattern used in stories and jokes, where part of the story is told three times, with minor variations. The first two instances build tension, and the third releases it by incorporating a twist.
Three is the smallest number required to create a pattern, so it's especially common in storytelling.The third of three siblings succeeds after their older siblings each failed. The protagonist is given three tests and receives the prize after the third. It's fairly unusual to find a folktale that does not incorporate the Rule of Three in some form.
Following on from the oral tradition, speech-writers have learnt the 'Rule of Three' — listen to a political speech — the points come in threes, from 'Ein Volk, Ein Reich, Ein Führer' to Tony Blair declaring 'Education, education, education' note . In persuasive or educational speaking, it also is a foundational concept: "Tell 'em what you're going to tell 'em; Tell 'em; Tell 'em what you told 'em" note
The Rule of Three is also used widely in comedy. Many popular jokes are based on three Stock Characters (e.g. Priest, Imam, Rabbi), all in the same situation. The first two react normally, the third does something ridiculous (but stereotypically in character). In Britain, Englishman, Irishman and Scotsman jokes denigrate either the Irishman as stupid or the Scotsman as a tightwad, while the Englishman is usually the Straight Man of the gag (Unless it's being told by the Scots or Irish. When an American tells it, Englishmen are stuffed shirts.) This is why most Americans have never heard of Wales. Another (geeky) variant is the engineer/physicist/mathematician series of jokes, however, these are virtually never considered offensive, largely because the stereotypes are often jokingly accepted by the members of those three groups. (e.g. The engineer is overly practical, the physicist makes large assumptions, and the mathematician comes up with a correct, but useless answer; these are played up for humorous effect, but have some valid basis.)
A more popular variation on the rule is to repeat the same joke or concept three times, but put a twist on the third one that makes it funny again. One version of this is The Triple, wherein a character lists three items - the first two logical and serious, and the third applying a twist or joke. For example, a character might say to a bald person, "Can I get you anything? Cup of coffee? Doughnut? Toupee?" (From The Dick Van Dyke Show.)
Alternatively, the twist can come during the second iteration (such as Chekhov's Skill) failing the first time it's used only to return to its original form on the third pass; this version tends to accompany Chekhov's tropes.
The Overly-Long Gag could be seen as a subversion of the Rule Of Three, because it fails to deliver the expected twist.
Sometimes, an event needs to be shown three times to establish that a variation to the norm is happening. The first time the audience sees this event, they see it happening a certain way, but they don't yet know that this is typical. The second time they see it, it is the same as the first. This establishes that this is the standard way that things always happen. The third time they see the event in question, it is different, so the audience knows that this is a deviation from the norm. For example, in The Shawshank Redemption, we see Red appear before the parole board three times. The first two appearances are practically identical. The third instance is different, indicating how Red changed after Andy left.
The trope is also incredibly common in fairytales and ghost stories that are part of oral tradition. The reason above is important, as audiences don't have a good idea of how this ghost/gnome/witch would typically behave, and it works well for building tension too. But another reason is that it's easy to remember. You get three times the story padding for only having to remember one short story and some minor variations. This makes the story easier to remember than non-repeating tales of the same length, both for professionals who collect as many stories as possible, and for people that pass a story on pretty much because they happened to remember it.
In art, there's a rule of thirds where putting items in the intersections between thirds-lines draws more attention and is more visually appealing than plonking them right in the center, which is considered boring. In design, particularly three-dimensional design such as shop displays, groups of three objects, or objects arranged to form a triangle, are considered most attractive to the eye.
The Rule of Three may be a subtrope of a more general psychological phenomenon, as threes are well-noted in all forms of culture. Films, books and plays come in trilogies. They have a Three-Act Structure, a Beginning, Middle and End. Counts of three elements are used widely in rhetoric, writing and myth: "Ready, aim, fire", "Veni, Vidi, Vici", "Lights, camera, action", "Reading, 'riting, 'rithmetic," "rhetoric, writing and myth". Just try and think about how many times you've heard the phrase "On the count of three..."
A constructed phrase such as "Veni, Vidi, Vici." that has three grammatically and logically connected elements is known as a Tricolon. When the three elements increase in length, it's a Tricolon Crescens.
This is why there are Power Trios and Terrible Trios.
Variations on this trope include uses of 5, 7, 12, and convenient multiples of 5 afterwards (i.e., 25 or 50, but not 35 or 70).
See also Basic Conflicts and other plot devices which often come in 3s or 7s, and Three Rules of Three, a wiki guideline. Not to be confused with 4, unless you're counting elements, bodily fluids, elite badasses, and other dimensions.note
