Tergeminous is a rare and complex word that refers to something being in three parts or triple. Its spelling can be a challenge, but its pronunciation can be broken down using IPA transcription as "ter-jem-uh-nuhs." The emphasis is on the second syllable, and the "g" is pronounced softly as "j." This word comes from the Latin term "tergeminus" which means "triplet" and is often used in biology to describe organisms or structures that come in threes.
Thrice double; threefold; three-paired.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.