The compound adjective 'three-headed' is a transparent modern English formation built from native Germanic elements. Its first component, 'three', descends from the Proto-Germanic numeral þrīz, which itself derives from the Proto-Indo-European root tréyes. The second component, 'head', originates from the Proto-Germanic haubudą, tracing back to the PIE root káput-. The suffix '-ed' is an ancient adjectival marker used to denote 'having the quality of'. The word's logic is entirely literal, combining these elements to describe a creature or entity possessing three heads, a concept prevalent in mythology and heraldry. Its formation follows the standard English pattern for creating descriptive compound adjectives, such as 'blue-eyed' or 'long-haired'.

Structure: three (number) + head (body part) + ed (adjective suffix) - three: From Old English þrīe, þrēo, of Proto-Germanic origin; meaning the number after two. - head: From Old English hēafod, of Proto-Germanic origin; meaning the upper part of the body containing the brain, eyes, and mouth. - ed: A suffix forming adjectives from nouns, indicating possession of a quality, from Old English -ede.