trigraph       
noun [ C ]  language specialized uk/ˈtraɪ.ɡrɑːf/ us/ˈtraɪ.ɡræf/
 三字母一音
three letters written together that, when spoken, make one sound
  This worksheet will help you introduce your pupils to the "igh" trigraph. 这页练习题将帮助你向学生介绍“igh”这个由三个字母组成的单音。 
 See also: digraph
 - More examples
 - Trigraphs are generally consonant groups or consonant and vowel combinations.
 - Words such as "rhythm" and "strength" contain not multiple consonants at all, but rather digraphs and trigraphs.
 - Geraldine Giraffe helps introduce the ARE trigraph by finding words that include that sound.
 - Katie-May Blight in Year One was able to tell me that she actually had a trigraph in her name.