E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The three golden balls. The Lombards were the first money-lenders in England, and those who borrowed money of them deposited some security or pawn. The Medici family, whose arms were three gilded pills, in allusion to their profession of medicine, were the richest merchants of Florence, and greatest money-lenders. (See BALLS.)
Roscoe, in his Life of Lorenzo de Medici, gives a different solution. He says that Averardo de Medici, a commander under Charlemagne, slew the giant Mugello, whose club he bore as a trophy. This club or mace had three iron balls, which the family adopted as their device.