Green’s Dictionary of Slang

three n.

1. (drugs) a $3 bag of heroin.

[US] ‘Kitty Barrett’ in D. Wepman et al. Life (1976) 52: I reached in my slide and came out with two boss threes.

2. (UK gang, also 3) code used in posts or messages, usually preceding a name, to indicate respect for an imprisoned person.

[UK]T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 3 code used in posts or messages, usually preceding a name, to indicate respect for an imprisoned person.

3. see finger n. (1b)

SE in slang uses

In derivatives

three-er (n.)

a three-year prison sentence.

[US]D. Runyon Runyon à la Carte 149: He is just after doing a three-er and it seems to leave some impression on him.
threesome (n.)

group sex involving three people of the same or mixed sexes.

[UK](con. 1930) E. Williams Emlyn 175: ‘Threesomes?’ ‘Good God, no.’.
[US]L. Kramer Faggots 178: We’ll find a big black stud and have a threesome.
[Aus]B. Humphries Traveller’s Tool 101: For the price of a couple of Baileys and Coke [...] they’d stack on a gleesome threesome.
[US] (ref. to 1960) in Walking After Midnight (1989) 88: [We] met some guy who was obviously cruising both of us [...] and we had a threesome.
[US](con. 1950s) Kennedy & Davies Boots of Leather (2014) 199: These [lesbian] women threw parties in which they tried threesomes and daisy chains.
[UK]Guardian 10 Sept. 16: The vases show us copulation in all its possible forms: vaginal, anal, fellatio [...] threesomes.
[Aus] 🌐 It then got on to the topic of threesomes, and after some gentle persuasion [...] she told me how an ex-boyfriend and her had a threesome one night with one of her best female friends!

In phrases

three-a-dayer (n.)

(US drugs) one who smokes opium three times a day; thus multiples, i.e. four-a-dayer.

[US]‘Boxcar Bertha’ Sister of the Road (1975) 115: Anna and Jake smoked twice a day. There were in the group, however, ‘three-a-dayers,’ and ‘four-a-dayers.’.
three-six-nine (n.)

(US black) misbehavior, mischief, ‘funny businesss’ .

[US]Lil Johnson ‘New Shave ’Em Dry’ 🎵 I’m gonna tell you-all women and please understand / Don’t start no three-six-nine with my man / ‘Cause if you do you’ll surely get down [?] wrong / I got a forty-four that puts your waters on.
[US]H. Morand ‘Don’t Start No Stuff’ 🎵 I’ll treat you nice, I’ll treat you kind / But I don’t want no three-six-nine.
three, three, twenty-one (n.)

(N.Z. prison) a puniashment for possession of drugs, i.e. three ays in solitary confinement, three days’ Loss of Remission and 21 days off privileges.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 189/1: three, three, twenty-one (also 3 3 21) n. a punishment for the possession of drugs.
three times ten (n.)

(UK Und.) a thorty-day sentence.

[US]Salt Lake Herald (UT) 19 Oct. 5/1: Jimmy gets off with the beak handing him three times ten.