engage in three to one (and bound to lose), v. — Green’s Dictionary of Slang

engage in three to one (and bound to lose) v.

[the image is of a conflict between the penis and two testicles and the vagina; the ‘loss’ is of semen]

to have sexual intercourse.

1793		‘As I Cam O’er the Cairney Mount’ in Burns Merry Muses of Caledonia (1965) 140: Altho’ he struck me three to one, / I stood my ground and receiv’ his fire.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 17: Arieter. To copulate; ‘to engage in three to one’.