ˈtrīˌgraf, -aa(ə)f, -aif, -ȧf noun
Etymology: tri- + -graph
1. : three letters spelling a single consonant, vowel, or diphthong
sch of schism, cch of bacchic, pph of sapphic, tth of Matthew, and eau of beau are trigraphs
2. : a cluster of three successive letters
THE is a high frequency trigraph