The word 'three-sided' is a transparent compound adjective formed within English. Its first element, 'three', descends from the Proto-Indo-European numeral trei-, which is the source for the number three across many Indo-European languages. The second element, 'side', originates from a Germanic root concerning the flank or extended surface of an object. The adjectival suffix '-ed' is appended to the noun 'side' to create the sense 'having sides', which is then modified by the numeral 'three'. The logic of its formation is purely descriptive, directly combining the count of sides with the concept of a side to specify a geometric or figurative property.

Structure: three (number) + side (edge) + ed (adjective suffix) - three: From Old English þrīe, þrēo, of Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as a numerical root. - side: From Old English sīde, of Germanic origin; related to Dutch zijde and German Seite, perhaps from a base meaning 'extending lengthways'. Functions as a nominal root meaning a bounding line or surface. - ed: A suffix forming adjectives from nouns, indicating 'having' or 'characterized by'. From Old English -ede.