Etymology
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trefoil (n.)
late 14c., type of clover, from Anglo-French trifoil (13c.), Old French trefueil "clover, clover-leaf," from Latin trifolium "three-leaved plant," from tri- "three" (see tri-) + folium "leaf" (from PIE root *bhel- (3) "to thrive, bloom"). As a type of ornamental figure in medieval architecture, early 15c.
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sampan (n.)

a word applied by Europeans to any small, light boat on the Chinese pattern, used on the coasts of East Asia, 1610s, from Chinese san pan, literally "three boards," from san "three" + pan "plank." In 16c. Spanish made it cempan; Portuguese had it as champana.

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Flavian (adj.)

1590s, pertaining to the three Roman emperors who reigned 69-96 C.E., the dynasty of (Flavius) Vespasian; see Flavius.

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tangent (n.)

1590, one of the three fundamental functions of trigonometry, from tangent (adj.). From 1650s as "a tangent line." Figurative use of on a tangent is from 1771.

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centillion (n.)

in France and U.S., "1,000 to the hundredth power," 1846, from centi- "one hundred" (in reference to the 100 groups of three zeroes it has beyond the first group of three zeroes) + ending from million, etc. Compare French centillion (by 1841). Generally used indefinitely for "a very large number." Related: Centillionth.

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stereo- 

before vowels stere-, word-forming element meaning "solid, firm; three-dimensional; stereophonic," from Greek stereos "solid" (from PIE root *ster- (1) "stiff").

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ferric (adj.)
1799, "pertaining to or extracted from iron," from Latin ferrum "iron" (see ferro-) + -ic. Especially of iron with a valence of three.
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triple (adj.)
early 15c., from Old French triple or directly from Latin triplus "threefold, triple," from tri- "three" (see tri-) + -plus "-fold" (see -plus). As a noun, early 15c., "a triple sum or quantity," from the adjective. The baseball sense of "a three-base hit" is attested from 1880. Related: Triply (adv.). Triple-decker is from 1940 of sandwiches and wedding cakes, 1942 of beds.
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middlemost (adj.)

early 14c., "being in or nearest the middle; being the middle one of three," from middle (adj.) + -most.

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tricuspid (n.)
1660s, from Latin tricuspidem (nominative tricuspis) "three-pointed," from tri- (see tri-) + cuspis "point" (see cusp).
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