Etymology
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three (adj., n.)

"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), from nominative plural of 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 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 is recorded by 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.

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threefold (adj.)
late Old English þrifeald; see three + -fold.
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trillium (n.)
1768, from Modern Latin trillium (Linnaeus, 1753), from Latin tri- "three" (see three). So called for its leaves and flower segments.
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trio (n.)
1724, "composition for three voices," from French trio (c. 1600), from Italian trio, from tri- "three" (see three); patterned on duo. Meaning "group of three persons" is from 1789.
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trine (adj.)
"threefold," late 14c., from Old French trine "triple, threefold" (13c.), from Latin trinus "threefold," from tres "three" (see three).
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trilogy (n.)
series of three related works, 1660s, from Greek trilogia "series of three related tragedies performed at Athens at the festival of Dionysus," from tri- "three" (see three) + logos "story" (see Logos).
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trilobite (n.)
extinct marine arthropod, 1820, from Modern Latin Trilobites (Walch, 1771), from Greek tri- "three" (see three) + lobos "lobe" (see lobe); so called because its body is divided into three lobes.
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thrice (adv.)
c. 1200, from Old English þriga, þriwa "thrice" (from þrie "three;" see three) + adverbial genitive -es, changed c. 1600 to -ce to reflect voiceless pronunciation.
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sitar (n.)
1845, from Hindi sitar, from Persian sitar "three-stringed," from si "three" (Old Persian thri-; see three) + tar "string," from PIE root *ten- "to stretch."
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