rule of three

Short definition from Wiktionary

Proper noun

1. 1
   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.
2. 2
   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.
3. 3
   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.
4. 4
   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: Specifically, the technique of having a setup in advance, a reminder to keep it fresh in the audience's memory, and then a payoff.
5. 5
   A religious tenet stating that the energy a person puts out into the world, positive or negative, will be returned to that person threefold.
6. 6
   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.
7. 7
   A religious tenet held by some Wiccans, stating that whatever energy, positive or negative, a person puts into the world will be returned threefold.