Green’s Dictionary of Slang

three balls n.

[the trad. three brass balls that hang outside a pawnshop. The stereotypical pawnbroker was Jewish. The three balls themselves supposedly come from the arms of the Medicis (although these were 6 red balls, rather than the three gold ones of pawnbroking) and were imported to London by the Lombard bankers and thence to the US]

1. [mid-19C+] (also Mr Three Balls, sign of the three balls, three-ball exchange, three ball joint, three-ball man, three-balls outfit) a pawnbroker or pawnbroker’s shop; also attrib.

2. [1930s+] (US black) a Jew.