The compound adjective "three-line" is a modern English formation following a straightforward number-noun pattern. The first element, "three," descends from the Proto-Indo-European root trei-, a fundamental numeral meaning "three," which evolved into its Old English form. The second element, "line," traces back to the Latin līnea, originally meaning a linen thread or cord, derived from līnum (flax). The compound logically denotes an object characterized by or consisting of three such lines, threads, or stripes, directly combining the ancient concepts of quantity and a linear mark.

Structure: three (number) + line (mark) - three: From Old English þrīe, þrēo, of Proto-Germanic origin; related to Dutch drie and German drei. Functions as a numeral root. - line: From Old English līne, from Latin līnea (linen thread, string, line). Functions as a noun root.