The compound adjective "three-foot-long" is a modern English formation built from three native Germanic elements. "Three" originates from the ancient Proto-Indo-European root for the numeral, which evolved consistently into the Old English form. "Foot," denoting the unit of length, descends from the PIE word for the body part, reflecting its use as a fundamental measure. "Long," describing linear extent, shares a deep PIE ancestry related to length. The compound's logic is transparently descriptive, combining a numerical quantity, a standard unit, and a dimension attribute to specify a precise physical measurement, following the common English pattern for creating compound modifiers.

Structure: three (numeral) + foot (noun) + long (adjective) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a numeral. - foot: From Old English fōt, from Proto-Germanic fōts, from Proto-Indo-European pṓds. Functions as a noun denoting a unit of measurement. - long: From Old English lang, long, from Proto-Germanic langaz, from Proto-Indo-European dlongʰos-. Functions as an adjective describing extent.