The compound adjective "three-word" is a transparent and relatively modern formation in English, directly combining two native Germanic elements. The first element, "three," descends from the Proto-Indo-European numeral tréyes, which is remarkably stable across Indo-European languages, giving rise to Latin trēs and Greek treîs. The second element, "word," originates from a PIE root meaning "to speak," which also produced Latin verbum (verb) and Greek rhētōr (orator). The logic of the compound is purely descriptive, following the common English pattern of number-noun combinations used attributively to specify a quantity, with the resulting term serving as a concise classifier for phrases, titles, or descriptions.

Structure: three (number) + word (unit of language) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a cardinal number. - word: From Old English word, from Proto-Germanic wurdan, from Proto-Indo-European werh₁- ("to speak, say"). Functions as a noun denoting a distinct unit of language.