The compound adjective 'three-prong' is a transparent combination of the numeral 'three' and the noun 'prong'. 'Three' descends from the Proto-Indo-European root trei-, a fundamental numeral that gave rise to equivalents across Indo-European languages. 'Prong' has a more obscure origin, likely from a Low German or Dutch source related to instruments for piercing or gripping. The literal combination, meaning "having three pointed projections," first described physical forks or tools. Its meaning broadened metaphorically to describe any strategy, attack, or plan that proceeds or is divided along three distinct lines or aspects, illustrating a common pattern where a concrete physical description evolves into an abstract conceptual one.

Structure: three (three) + prong (pointed projection) - three: From Old English þrīe, þrēo, of Proto-Germanic origin; related to Dutch drie and German drei. Functions as a numeral root. - prong: From Middle English pronge, likely from a Germanic root meaning 'to press, pinch'. Functions as a noun root meaning a pointed projecting part.