rule of three
proper noun
Definitions
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 \frac ab=\frac cd then d=\frac{bc}a.
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) 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.
A religious tenet stating that the energy a person puts out into the world, positive or negative, will be returned to that person threefold.