trigraph

noun

tri·​graph ˈtrī-ˌgraf 

1

: three letters spelling a single consonant, vowel, or diphthong

eau of beau is a trigraph

2

: a cluster of three successive letters

the, ion, and ing are high frequency trigraphs

trigraphic

(ˌ)trī-ˈgra-fik 

adjective

Word History

First Known Use

circa 1836, in the meaning defined at sense 1