Origin and history of triad

"1 more than two; the number which is one more than two; a symbol representing this number;" Old English þreo, fem. and neuter (masc. þri, þrie), from Proto-Germanic *thrijiz (source also of Old Saxon thria, Old Frisian thre, Middle Dutch and Dutch drie, Old High German dri, German drei, Old Norse þrir, Danish tre). This is from PIE root *trei- "three" (source also of Sanskrit trayas, Avestan thri, Greek treis, Latin tres, Lithuanian trys, Old Church Slavonic trye, Irish and Welsh tri "three").

3-D (adj.) is attested by 1952, abbreviation of three-dimensional (1878); the three dimensions were so called from late 14c. The clothing three-piece suit is recorded by 1897. The three-martini lunch is attested from 1972.

Three cheers for ______ is recorded from 1751. Three-ring circus is recorded by 1898. Three musketeers translates French les trois mousquetaires, title of the 1844 novel by Alexandre Dumas père.

Three-sixty "complete turnaround" is from 1927, originally among aviators, in reference to the number of degrees in a full circle.

in geology, in reference to the lower division of the Mesozoic, 1841, coined 1841 by German geologist Friedrich August von Alberti (1795-1878), from Trias already used in German in geology for "period preceding the Jurassic," from Greek trias "triad, the number three" (see triad). So called because it is divisible (in Germany) into three groups.