The compound "three-family" is a modern English formation, combining the cardinal number "three" with the noun "family." The word "three" has ancient roots, traceable through Proto-Germanic to the Proto-Indo-European tréyes, a fundamental numeral. "Family" entered English via Old French from Latin familia, which originally referred to the entire household, including servants under the authority of the paterfamilias. The compound logically and transparently denotes a residential building designed to accommodate three such distinct domestic units, reflecting a practical architectural and social classification.

Structure: three (number) + family (group) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a numerical quantifier. - family: From Latin familia (household, servants), from famulus (servant). Functions as the core noun denoting a social unit.