The Rule of Three is a principle that exists in many fields, but in narrative construction, it states that patterns in groups of three are the most satisfying and, as Sarah puts it, “memorable.” Throughout the novel, elements are repeatedly introduced as parts of a set of three. Sarah explains to West and Hannah, “Basically, the idea is that things are more memorable if it’s in a group of three” (62). The funhouse is full of elements that work in sets of three. The symbolism of the wishbone, the deck of cards, and the compass is one of the most salient examples. The Deltas also realize that most rooms offer three different exits or three solutions to a riddle, allowing them to look for alternatives when they are stuck. Triangles are also a recurrent visual signal of the rule of threes in the house, with triangular windows, mirrors, and drawings appearing in nearly every room the characters explore.

The most significant example of the rule of threes, however, is the characters themselves. There are three friends, Sarah, West, and Hannah, who call themselves the Deltas because of their common love of math, and three Stein brothers, who are triplets.