A group of three things; a trio, threesome or tierce.
Arranged in groups of three.
Having three variables.
To the base three.
Containing, or consisting of, three different parts, as elements, atoms, groups, or radicals, which are regarded as having different functions or relations in the molecule.
Made up of three things.
The practice of three people having sex, i.e. having threesomes.
The practice of two people having sex while a third person watches (and may or may not take part).