three: [OE] Three goes back to a prehistoric Indo- European *trejes, which also produced Greek treis, Latin trēs, Russian tri, Sanskrit tráyas, etc. Its Germanic descendant was *thrijiz, which has evolved into German drei, Dutch drie, Swedish and Danish tre, and English three. Amongst three’s many relations in English are treble, trellis [14] (etymologically something made from ‘three threads’), trinity, trio [18], triple, tripod [17] (etymologically something with ‘three feet’), trivial and possibly also travail and tribe. => drill, testament, third, travail, treble, trellis, tribe, trinity, trio, triple, tripod, trivet, trivial
three (adj.)
Old English treo, fem. and neuter (masc. tri, trie), from Proto-Germanic *thrijiz (cognates: Old Saxon thria, Old Frisian thre, Middle Dutch and Dutch drie, Old High German dri, German drei, Old Norse trir, Danish tre), from nominative plural of PIE root *trei- "three" (cognates: Sanskrit trayas, Avestan thri, Greek treis, Latin tres, Lithuanian trys, Old Church Slavonic trye, Irish and Welsh tri "three").
3-D first attested 1952, abbreviation of three-dimensional (1878). Three-piece suit is recorded from 1909. Three cheers for ______ is recorded from 1751. Three-martini lunch is attested from 1972. Three-ring circus first recorded 1898. Three-sixty "complete turnaround" is from 1927, originally among aviators, in reference to the number of degrees in a full circle. Three musketeers translates French les trois mousquetaires, title of the 1844 novel by Alexandre Dumas père.