The compound adjective "three-decade" is a modern English formation built from two distinct numerical roots. The first element, "three," descends from the Proto-Indo-European root trei-, which is the ancient source for the concept of "three" across many languages. The second element, "decade," originates from the Greek deka, meaning "ten," via Latin and French, specifically referring to a set or period of ten years. The compound logically combines these elements, following the common English pattern of number-noun compounds (e.g., two-hour, five-page), to create a precise term denoting a span of thirty years. Its formation reflects a transparent, analytical approach to describing duration, where the ancient roots for counting and measurement are directly applied to modern chronological contexts.

Structure: three (numeral) + decade (noun) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a numeral quantifier. - decade: From Middle French décade, from Late Latin decadem (accusative of decas), from Greek dekas (a group of ten), from deka (ten). Functions as the noun unit being quantified.