Rule of Threes

Either of 2 guidelines for staged reconstructive surgery of the hand, to determine whether a digit should be amputated
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References in classic literature ?
And now it's open, we are going out by Rule--the Rule of Three."
"He thought he saw a Garden-Door That opened with a key: He looked again, and found it was A Double Rule of Three:
It sounds like a sum in the rule of three. The answer should give us the--But halloo!
The world is filled with the proverbs and acts and winkings of a base prudence, which is a devotion to matter, as if we possessed no other faculties than the palate, the nose, the touch, the eye and ear; a prudence which adores the Rule of Three, which never subscribes, which never gives, which seldom lends, and asks but one question of any project,--Will it bake bread?
As difficult to decipher as a hieroglyphic inscription to the clerks, the vocation of the secretary and his usefulness were as plain as the rule of three to the self-interested.
You can work it out by Fractions or by simple Rule of Three, But the way of Tweedle-dum is not the way of Tweedle-dee.
ORLANDO--The rule of threes that has been used to identify patients at risk of hereditary melanoma who may be candidates for genetic testing may be modified soon, according to Sancy Leachman, MD, PhD, professor and chair of the department of dermatology, Oregon Health & Science University, Portland.
In the interview, she discussed a soon-to-be-published literature review that builds upon the rule of threes and suggests a strategy for deciding which patients should be considered for genetic testing, and includes "a suggested list of genes" that should be used in these different subsets of patients.