tres

tres declines, and it is the last numeral that does. Masculine and feminine share tres in the nominative and accusative (with an older accusative tris), the neuter is tria, the genitive is trium for all genders and the dative and ablative are tribus. From quattuor up to centum every cardinal number is indeclinable, so unus, duo and tres are the whole of the work.

The ordinal is tertius, "third", and the distributive terni, "three each". The combining form tri- is everywhere: triduum (a space of three days), triumvir, tribunus, trini. Descendants: Italian tre, Spanish tres, French trois, and English "trio", "triple" and "trinity" through the same stem.