What is another word for "trigon"?

Roget's Thesaurus (1911) lists these synonyms for "trigon": cube, leash, power, root, shamrock, ternary, ternion, tertiary, third, three, tierce, trefoil, trey, triad, triangle, trident, triform, trinal, trinity, trinomial, trio, triple, triplet, tripod, trireme, triskele, triskelion, triune, triunity.