rule of threes

Either of 2 guidelines for staged reconstructive surgery of the hand, to determine whether a digit should be amputated

rule of threes,

n a procedure for approximating the location of the transverse process of a thoracic vertebra by using that same vertebra's spinous process as a landmark.
References in classic literature ?
And now it's open, we are going out by Rule--the Rule of Three.
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.