The compound adjective "three-night" is a transparent combination of two ancient Germanic elements. "Three" descends from the Proto-Indo-European root tréyes, a fundamental numeral that has remained remarkably stable in form and meaning across millennia into English. "Night" originates from the PIE nókʷts, referring to the dark period, a concept essential to early human timekeeping. The modern compound follows a straightforward English pattern of number-noun compounding (e.g., two-day, four-hour) to create a precise adjectival descriptor for duration, directly linking the countable unit ("night") with its specific quantity ("three").

Structure: three (number) + night (period of darkness) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a cardinal number. - night: From Old English niht, from Proto-Germanic nahts, from Proto-Indo-European nókʷts. Functions as a noun denoting the period of darkness.