A tricolon is a series of three parallel elements — three words, three phrases, or three clauses of matching construction. Three is the smallest number that establishes a pattern and completes it in the same breath, which is why the figure appears in almost every tradition of public speech.
Two items read as a comparison. Four begin to read as a list. Three reads as a shape.
Lincoln at Gettysburg: “government of the people, by the people, for the people.” Three prepositions, one noun, nothing else changes. The variation is so slight that the ear hears a single idea turned through three positions rather than three separate claims.
A tricolon whose elements grow in length or force as they go is a tricolon crescens — the arrangement that makes the third item land hardest.
The tricolon is the most reliable rhythmic unit in English prose, and its reliability is the problem as well as the point. Because three parallel items feel complete, a tricolon can make an arbitrary selection seem exhaustive. Any three things presented in matching form acquire the air of being the three things.
That is worth watching for in argument. A writer who lists three causes in parallel has not shown there are only three, and the grammar will not tell you whether the fourth was omitted for rhythm or because it was inconvenient.
The device also degrades quickly under repetition. One tricolon in a paragraph is structure; three in a paragraph is a tic, and the reader starts hearing the pattern rather than the content. It survives in speech far better than on the page, because a listener cannot go back and count.
Where a tricolon's three elements are also offered as the complete set of options, it stops being a figure of arrangement and starts doing the work of an argument — at which point the question is whether the set is genuinely exhaustive.
