[runevault]

People inherently pattern match everything, looking for how it fits their worldview. Three points either lets you see the pattern and see everything fits it, or gives you enough setup to create payoff.

If you only do single setup plus payoff the audience may instead try to make the two fit together. If you give them two points of reference, it allows them to prebuild an assumed pattern with less work from the story/music/whatever for you to either play within or subvert.

An example of using just two to build patterns in the audience (but doesn't give a chance to subvert expectations per se) is cutting between shots in movies. The audience inherently tries to deduce the meaning of going from one image to a second. David Mamet has a great book, 'On Directing Film', that is not very long and talks about the shot transition version of this in great detail. You can use the ideas there and expand it to three to get into the ideas I've talked about above.