Ternary is the right word. Ternary numbers or logic have 0,1,2 or true, false, and something else. Ternary functions just have three arguments that could be bin

Ternary is the right word. Ternary numbers or logic have 0,1,2 or true, false, and something else. Ternary functions just have three arguments that could be binary logic, similar to a second order polynomial. Ternary comes from the Latin root word ternarius (composed of three things) which contains the root word ter- or tern- which is an adverb variation of the original root terti- meaning 3rd. Same with binary, it comes from the adverb variation bi- or bin- meaning second. Bi- being derived from the original Greek dy- or di-. However, ternary is more correct than trinary because in the binary, we are not using the Latin root for the cardinal two, duo-. We use the adverb version of two, bi-. Trinary just sounds better to some people because tri- and bi- rhyme. For a numerical system composed of four components, we use quarternary which also uses the adverb form of 4, quarter(n)-, instead of the cardinal form, quadr(i)-. Personal opinion, saying trinary makes you sound like you don't have any formal education.