The compound adjective 'three-room' is a modern English formation following a transparent Germanic pattern of number-noun compounding, akin to 'two-story' or 'five-page'. Its first element, 'three', descends from the ancient Proto-Indo-European root tréyes, a cardinal number that has remained remarkably stable in meaning across millennia into the Germanic lineage. The second element, 'room', originates from a Proto-Indo-European concept of openness or free space (rewh₁-), which in Old English narrowed to signify a measurable area or chamber. The compound's literal logic is perfectly straightforward, directly describing a property—such as an apartment or house—that contains three such discrete chambers or living spaces.

Structure: three (number) + room (chamber) - three: From Old English þrīe, þrēo, from Proto-Germanic þrīz, from Proto-Indo-European tréyes. Functions as a numerical quantifier. - room: From Old English rūm ("space, extent, chamber"), from Proto-Germanic rūmaz, from Proto-Indo-European rewh₁- ("to open, wide"). Functions as a noun denoting an enclosed space or chamber within a building.