The compound adjective 'three-car' is a modern, transparent formation from two ancient roots. The numeral 'three' descends from the Proto-Indo-European root trei-, a fundamental counting term preserved across major language families, from Greek treis to Latin tres. The noun 'car' has a more winding journey, originating from a Celtic word for a wagon or cart, borrowed into Latin as carrus, which then passed into Old Northern French and finally into Middle English. The logic of the compound is purely descriptive and compositional, directly combining the quantity 'three' with the object 'car' to specify a unit comprised of three such vehicles, most commonly applied to trains or connected road vehicles.

Structure: three (number) + car (vehicle) - three: From Old English þrīe, 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 quantifier. - car: From Middle English carre, from Anglo-Norman French carre, based on Latin carrum, carrus (wheeled vehicle), of Celtic origin. Functions as the noun being quantified.