trei- / Indo-European roots
Examples of words with the root trei-: attest, contest, detest, Hermes Trismegistus, obtest, protest, riding, sesterce, sitar, teapoy, tercel, tercet, termolecular, tern, terpolymer, tertian, tertiary, testament, testify, third, thirteen, thirty, three, thrice, tierce, trammel, trecento, tredecillion, trench, trephine.
▲ Derivatives include three, trio, testicle, detest, trinity.
I. Nominative plural form *treyes.
1. a. three, thrice; thirteen, thirty from Old English thrīe, thrēo, thrī, three, with its derivatives thrīga, thrīwa, thrice, thrītig, thirty, and thrēotīne, thirteen (-tīne, ten; see dekm̥); b. trillium from Old Swedish thrīr, three. Both a and b from Germanic *thrijiz.
2. trey; trammel, trecento, trephine, triumvir, trocar from Latin trēs, three.
3. triskaidekaphobia from Greek treis,tris, three.
II. Zero-grade form *tri-.
1. Suffixed form *tri-tyo-. a. (i) third from Old English thrid(d)a,thirdda, third; (ii) riding2 from Old Norse thridhi, third. Both
(i) and (ii) from Germanic *thridjaz, third; b. tercel, tercet, tertian, tertiary, tierce; sesterce from Latin tertius, third.
2. Combining form *tri-. a. tri-, tribe, trio, triple from Latin tri-, three; b. tri-; triclinium, tricrotic, tridactyl, triglyph, tritone from Greek tri-, three; c. Trimurti from Sanskrit tri-, three.
3. triad from Greek trias, the number three.
4. trichotomy from Greek trikha, in three parts.
5. trierarch from Greek compound triērēs, galley with three banks of oars, trireme (-ērēs, oar; see erə-)
6. Suffixed form *tri-to-. tritium from Greek tritos, third.
7. Compound form *tri-pl-, "threefold" (*-pl-, < combining form *-plo-, -fold; see pel-2) triploblastic from Greek triploos, triple.
8. Compound form *tri-plek-, "threefold" (*-plek-,-fold; see plek-) triplex from Latin triplex, triple.
9. Compound form *tri-st-i-, "third person standing by" (-st-, standing; see stā-) testament, testimony, testicle, testis; attest, contest, detest, obtest, protest, testify from Latin testis, a witness.
10. sitar, teapoy from Persian si, three.
III. Extended zero-grade form *tris, "thrice"
1. tern2; termolecular, terpolymer from Latin ter, thrice.
2. trisoctahedron, Hermes Trismegistus from Greek tris, thrice.
3. Suffixed form *tris-no-. trine, trinity from Latin trīnī, three each.
IV. Suffixed o-grade form *troy-o-. troika from Russian troje, group of three.
[Pokorny trei- 1090.]