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.