The compound adjective 'three-tier' is a modern English formation that transparently combines the cardinal number 'three' with the noun 'tier'. The number 'three' has a stable lineage from Proto-Indo-European, preserved in most major language families. The word 'tier' entered English from Old French, where it originally denoted a sequence or order, likely derived from the action of drawing things into a line. When combined, the morphemes retain their literal meanings, creating a term that logically describes a structure organized into three distinct levels or ranks, commonly applied to systems, cakes, or organizational models.

Structure: three (number) + tier (level) - three: From Old English þrīe, þrēo, of Proto-Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as a cardinal number. - tier: From late Middle English, from Old French tire 'sequence, order', likely from tirer 'to draw, pull'. Functions as a noun meaning a row, rank, or level.