
1) ABC or DEF 2) Cluster of three letters 
Found on 
https://www.crosswordclues.com/clue/trigraph

1) Letter 
Found on 
https://www.crosswordclues.com/clue/trigraph
 [orthography]
[orthography]  A trigraph (from the τρεῖς, treîs, `three` and γράφω, gráphō, `write`) is a group of three letters used to represent a single sound or a combination of sounds that does not correspond to the written letters combined. ==Latin-script trigraphs== For example, in the word schilling, the trigraph sch represents the vo...
Found on 
http://en.wikipedia.org/wiki/Trigraph_(orthography)

combination of three letters for one sound
Found on 
http://phrontistery.info/t.html

• (n.) Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.
Found on 
http://thinkexist.com/dictionary/meaning/trigraph/

A combination of three symbols or letters to indicate a single sound phonetically. For instance, the
Found on 
http://www.encyclo.co.uk/local/22385

alternative representation for C++ representation characters that doesn't exist in every national character set, such as {, }, [, ], and #: ??<, ??>, ??(, ??), and ??=. TC++PL C.3.1.
Found on 
http://www.stroustrup.com/glossary.html

trigraph A combination of three letters denoting a simple sound; such as, eau in plateau, or igh in high or thigh.
Found on 
http://www.wordinfo.info/words/index/info/view_unit/2724/72

 three letters representing one phoneme: high; fudge. 
Found on 
https://www.encyclo.co.uk/local/20815

Group of three letters representing a single sound or phoneme, for example 
ous in `dangerous`
Found on 
https://www.encyclo.co.uk/local/21221
  No exact match found.