Three letters forming a single sound

• Cluster of letters, like "sch"
• Cluster of three letters
• Sound represented by three letters

Wikipedia Word definitions in Wikipedia
A trigraph (from the , 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.

Wiktionary Word definitions in Wiktionary
n. (context linguistics English) A specific sequence of three letters, especially one used collectively to represent a single phoneme.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Trigraph \Tri"graph\, n. [Pref. tri- + -graph.] Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.