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.