Etymology
Advertisement
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.

Related entries & more 
Advertisement
threefold (adj.)
late Old English þrifeald; see three + -fold.
Related entries & more 
Three Rs (n.)

1824; said to have been given as a toast by Sir William Curtis (1752-1829), a beloved lord mayor of London in the 1820s, who seems to have been a figure of fun to whom many mangled phrases were attributed. Among the toasts he is alleged to have given at public dinners were "The Female Ladies of London;" "The three C's—Cox, King, and Curtis;" and "The three R's—Reading, Writing, and Rithmetic."

It has been very much the fashion amongst a class of persons to attribute to Sir W. C. ... a vulgarity and ignorance of speech which are by no means consistent with his character and conduct. The worthy and hospitable baronet has a rapid mode of speech, but it is always correct ; and although some eccentricities are mixed up in his composition, he is highly honourable, and has been a very useful member of society, particularly to his London constituents. [The Mirror, Jan. 29, 1825]

After listing some examples, the article continues:

It is, however, very certain, that at a city festival some years ago, having indulged very freely, he fell asleep, when some wag, choosing to consider him dead, wrote his epitaph, which was found next morning pinned to the baronet's dress coat:—
"Here lies the great Curtis,
Of London, Lord May'r:
He's left this here world,
And gone to that there."
Related entries & more 
trillium (n.)
1768, from Modern Latin trillium (Linnaeus, 1753), from Latin tri- "three" (see three). So called for its leaves and flower segments.
Related entries & more 
Advertisement
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.
Related entries & more 
trine (adj.)
"threefold," late 14c., from Old French trine "triple, threefold" (13c.), from Latin trinus "threefold," from tres "three" (see three).
Related entries & more 
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).
Related entries & more 
ter- 
word-forming element meaning "thrice, three times," from Latin ter "thrice," from *tris-, from root of three. Compare Latin tertius "third."
Related entries & more 
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."
Related entries & more