Webster's Revised Unabridged Dictionary
Pref. tri-, + -phthong, as in diphthong,: cf. F. triphthonque,
Chambers's Twentieth Century Dictionary
Gr. treis, three, phthongos, sound.
A TRIPHTHONG is produced by running together three vowel sounds in the same syllable.
"Orthography" by Elmer W. Cavins
The Serbian language being phonetic does not employ double consonants, diphthongs or triphthongs.
"Hero Tales and Legends of the Serbians" by Woislav M. Petrovitch