Green’s Dictionary of Slang

three legs n.

the legs plus a penis (or the penis and testicles).

1937

R. Johnson ‘Stones in My Passway’ 🎵 I got three legs to truck on, boys please don’t block my road / I’ve been feeling ashamed about my rider, babe I’m booked and I got to go.

In compounds

three-legged (adj.)

of the penis, erect.

1970

‘Troy Conway’ Cunning Linguist (1973) 59: All that served to do was to make me ramrod up in a hurry. Soon I was literally three-legged and the hidden girl between my knees was quick to seize a good opportunity when she saw one.

In derivatives

three-leggedness (n.)

(US) a state of being erect.

1970

‘Troy Conway’ Cunning Linguist (1973) 81: He had the worst case of three-leggedness and the Great Stone Ache I have ever seen.