The compound adjective "three-year" is a modern English formation that transparently combines two ancient, foundational elements. The first element, "three," descends from the Proto-Indo-European root tréyes, a cardinal number that has remained remarkably stable in meaning across millennia through its Germanic lineage. The second element, "year," originates from the Proto-Indo-European yóh₁r̥, meaning "year" or "season," a term essential for measuring cyclical time. The hyphenated structure follows a common English pattern for creating attributive adjectives that specify duration, where the numeral modifies the singular form of the time unit to describe a period or an event lasting that exact length of time.

Structure: three (cardinal number) + year (unit of time) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a cardinal number. - year: From Old English ġēar, from Proto-Germanic jērą, from Proto-Indo-European yóh₁r̥. Functions as a noun denoting a unit of time.