trigraph

Three letters spelling one consonant, vowel, or diphthong, such as Sch in Schiller or igh in high or thigh.

A group of three letters, especially of frequent occurrence in a given language, as the or ing in English or gli in Italian.

Any combination of three letters of an alphabet.

A combination of three letters to represent one sound; a triphthong, as eau in beau.

Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.