rule of three (arith.), that rule which directs, when three terms are given, how to find a fourth, which shall have the same ratio to the third term as the second has to the first; proportion. see proportion, 5 (b).. see rule
(n.) = regla de tres, la. Ex:In cases where more than 3 authors share responsibility for the creation of a work, the so-called Rule of Three dictates that the catalogue should provide added entry under only the 1st author named.