rule of three

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun If two ratios are equal, then the denominator of the second equals its numerator times the reciprocal of the first ratio: if then .
• proper noun 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.
• proper noun 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.
• proper noun religion, Wicca A religious tenet stating that whatever energy a person puts out into the world, be it positive or negative, will be returned to that person three times.