Pace Webster, a sequence of three letters is a trigraph but not strictly a triphthong. A sequence of three vowel sounds forming a single syllable is very rare, but examples may be found.

In Chinese the triphthong iao occurs, as in jiao.

In the older RP variety of British English, the vowels in fire and power could be triphthongs, [aI@] and [aU@] (where @ represents the un-ASCII-able schwa symbol).

I don't know whether adieu was two syllables in Old French. If it was, then it contained a triphthong, in which the components were pronounced actually as written, i-e-u, within a single syllable.

Triph"thong (?), 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.

 

© Webster 1913.

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