rule of three

(mathematics education) A rule stating that, if two ratios are equal, then the denominator of the second equals its numerator times the reciprocal of the first ratio: if a/b=c/d then d=(bc)/a.

(medicine) A rule for clinical trials used for determining the rate of adverse side effects when no such side effects present during the course of the trial.

(writing, rhetoric) A rule which states that things which come in groups of three are inherently funnier or more effective than things which come in groups of other sizes.

(writing, rhetoric) Specifically, the technique of having a setup in advance, a reminder to keep it fresh in the audience's memory, and then a payoff.

(religion, Wicca) A religious tenet stating that the energy a person puts out into the world, positive or negative, will be returned to that person threefold.

(programming) A rule of thumb in the C++ programming language, stating that any class that defines a destructor, a copy constructor, or a copy assignment operator should define all three of them.