threes

An ordinary regular plural: the numeral, used as a noun, simply takes -s. It counts anything a single three can name, such as written digits, cards with three pips, children aged three, or lengths of three units. Several of these plurals have hardened into words of their own that live only in the plural, like the poker pair and the basketball shots.

A clipped form of three-pointer, the shot from beyond the arc that counts for three rather than the two scored inside it. Commentary pairs it with verbs of striking, as players hit or bury threes. It also names the spot the shot came from, as in threes from the corner. It almost always appears in the plural, as a tally of a player's or team's long-range scoring.

Plural in form but naming one stretch of a child's life, it behaves like the decade words twenties and thirties. A child is in his threes, never in his three. The usual frame is in plus a possessive, which covers the whole year between the third and fourth birthdays rather than a single date.

Card-table shorthand names a pair by its rank in the plural. Threes means two cards of that rank, just as sevens or kings would. As pairs go it sits near the bottom and loses to any higher pair at showdown. It is slang, so it fits table talk and commentary, while a careful hand description would say a pair of threes.

British prison talk numbers the landings of a wing from the ground up. The ground level is the ones, so the threes are the cells two flights above it. The landing is something you are on, so the phrase is on the threes, never in them. It is slang, and it belongs to the speech of the prison rather than to its official records.