Divinity in Odd Numbers
Falstaff tells us (in the Merry Wives of Windsor, v. 1) that this divinity affects “nativity, chance, and death.” A Trinity is by no means confined to the Christian creed. The Brahmins represent their god with three heads; the Greeks and Romans had three Graces, three Fates, three Furies, and a three-fold Hecate. Jupiter had his three thunderbolts, Neptune his trident, and Pluto his three-headed dog. The Muses were three times three. Pythagoras says God is threefold—“the beginning, middle, and end of all things.” Then, again, there are five features, five parts to the body, five vowels, five lines in music, five acts to a play, etc.; seven strings to a harp, seven planets (anciently, at any rate), seven musical notes, etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894