The compound adjective "three-hour" is a modern English formation that transparently combines two ancient elements to denote a specific duration. The numeral "three" originates from a robust Proto-Indo-European root trei-, which gave rise to the concept of triplicity across numerous languages, including its Old English form. The word "hour" has a more complex journey, stemming from the Greek hōrā, which originally meant a period or season of the year, a meaning that narrowed significantly as it passed into Latin and then Old French. The logic of the compound is purely arithmetic and descriptive, directly applying the quantifying numeral to the standardized unit of time to create a single adjectival unit meaning "having the length of three hours."

Structure: three (quantity) + hour (unit of time) - three: From Old English þrīe, þrēo, of Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as a cardinal numeral quantifying the noun. - hour: From Old French hore, via Latin hōra from Greek hōra 'season, hour'. Functions as a noun denoting a unit of time.