Etymology
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ter- 
word-forming element meaning "thrice, three times," from Latin ter "thrice," from *tris-, from root of three. Compare Latin tertius "third."
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tri- 
word-forming element meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek treis, trias "three" (see three).
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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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