triphthong

n. [ Pref. tri- + -phthong, as in diphthong: cf. F. triphthonque. ] (Orthoepy) A combination of three vowel sounds in a single syllable, forming a simple or compound sound; also, a union of three vowel characters, representing together a single sound; a trigraph; as, eye, -ieu in adieu, -eau in beau, are examples of triphthongs. [ 1913 Webster ]