The compound adjective "three-drug" is a modern, transparent formation in English, created by directly combining the cardinal number "three" with the noun "drug." The number "three" traces its lineage back to the Proto-Indo-European root trei-, which gave rise to the numeral across many Indo-European languages, signifying a specific quantity. The word "drug" entered English via Romance and Germanic trade languages, originally associated with dried medicinal plants. In contemporary medical and pharmacological contexts, "three-drug" logically specifies a therapeutic protocol or combination product involving precisely three distinct pharmaceutical agents, reflecting a straightforward descriptive compounding common in technical jargon.

Structure: three (number) + drug (substance) - 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 numerical quantifier. - drug: From Middle English drogge, from Old French drogue, possibly from Middle Dutch droge, meaning 'dry' (as in dry goods). Functions as the core noun denoting a medicinal or psychoactive substance.