While not necessarily a hard and fast commandment, the rule of three provides structure for many components of a script. An audience can't read along, re-read, or rewind a movie in a theater, nor can they with a play. The rule of three provides assistance.
There is a lilt of a series of three rolling off the tongue, such as "Tom, Dick, and Harry"; "coffee, tea, or me"; or "big, bad, and beautiful." We are conditioned, upon hearing a sequence begin, to mentally complete that sequence, based on our own experiences and memories. It's sort of like the old psychiatric drill of word association. One person says "black" and another person responds with "white." Start a sequence with "a" and "b," and a natural mental process is to complete it with "c."
Jokes are the most prominent, and probably the most often-used, recipients of the rule of three. A standard formula for comedy writing is hit 'em with something, hit 'em with something else, and then hit 'em with the punch line.
The first two components set up the audience and the third surprises them, to their delight.
The initial reference establishes the situation, sets up the subject matter, and lays the foundation of the premise.
The second component then backs up the first, continuing the subject matter, or creating a sequence of some sort in the audience's mind.
The third component completes the set. It may establish a satisfactory conclusion, it may bring about a surprising end, or it may begin a new sequence again.
Character establishment and audience recognition are other examples where the rule of three emerges. When a new character is introduced to the audience, by the time that character's name is said (or presented) three times, the audience's recognition is established.
It's confusing to try to follow a storyline when characters' names are being bandied about without the audience having first matched up those names with faces. But also remember that there are many very creative ways of reaching the same goal.
Similarly, as plot lines unfold, the audience will follow along, hearing some aspect the first time, then having a secondary booster of the idea to create familiarity, and finally firmly entrenching the aspect with a third reference. From that point on, any further references to the subject or aspect will be quickly recognized, along with its significance to the rest of the show.
