Green’s Dictionary of Slang

(UK Und./gambling) the ‘three-card trick’, usu. played on the street, the ‘lady’ being a solitary queen alongside two nondescript cards.

1935		X. Petulengro Romany Life 245: The Finding-the-Lady Three-card-Trick is another game at which you cannot win.
1989		J. Morton Lowspeak.
2003		D. Swierczynski Complete Idiot’s Guide to Frauds, Scams, and Cons 96: Three-Card Honte [sic] In Great Britain, it’s known as ‘Find the Lady’.