Definition of trigraph in English:
 trigraph
noun ˈtrʌɪɡrɑːfˈtraɪˌɡræf
A group of three letters representing one sound, for example German sch-.
 Example sentencesExamples
-  If three letters together represent a single sound, they constitute a trigraph, such as tch in catch and sch in schmaltz.
 -  Both digraphs and trigraphs in French have their origin in history and phonology.
 -  Similarly, to take the consonant trigraph t c h, the teacher can explain that in this word the three letters together ‘say’ the sound.