/ Dictionary / Index R rule of three: Meaning and Definition of Find definitions for: rule' of three' Pronunciation: [key] — Math. Math. the method of finding the fourth term in a proportion when three terms are given. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. rule of the road rule of thumb Related Content Daily Word Quiz: sequester Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language