triphthong
A monosyllabic vowel combination usually involving a quick but smooth movement from one vowel to another that passes over a third one.
In Lang Belta vowels usually appear preceding or following a consonant (see previous examples), and as of now no occurrence of diphthongs or triphthongs has been attested.
A disyllabic sequence of a vowel, a semivowel, and a vowel.
Before moving on to a solution, it may be worth emphasizing that nouns of the صلوه type originally contained a triphthong. […] In addition to evidence from the Safaitic inscriptions, the discovery of the Graeco-Arabic inscription A1 (Al-Jallad and al-Manaser 2015), where the verb ’tw ‘he came’ is written αθαοα /’atawa/, confirms this to be the case for Old Arabic as well.
Where it comes from
From the Ancient Greek τρίφθογγος (tríphthongos, “triphthong, having three voices”), from τρία (tría, “three”) + φθόγγος (phthóngos, “sound”).
Adapted from the Wiktionary entry triphthong (CC BY-SA 4.0).