Noun
Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.
Source: Webster's dictionaryThe same combination of letters might not be the digraph or trigraph but might be just the non-nasal vowel followed by the consonant at full weight. Source: Internet
Usually a trigraph is a double digraph, but there are a few exceptions: tizennyolc ("eighteen") is a concatenation of tizen + nyolc. Source: Internet