the method of finding the fourth term in a proportion when three terms are given.
Origin
1585-95
Examples from the web for rule of three
If public notice, decide whether to use category ranking or rule of three.
Candidates cannot be eliminated after three considerations as with the rule of three.
Candidates are selected off of the eligible list using the rule of three.
British Dictionary definitions for rule of three
rule of three
noun
1.
a mathematical rule asserting that the value of one unknown quantity in a proportion is found by multiplying the denominator of each ratio by the numerator of the other