trigram vs written word

trigram and written word both are nouns.

Word	Noun	Adjective	Verb	Adverb
trigram	Yes	No	No	No
written word	Yes	No	No	No

As nouns, written word is a hypernym of trigram; that is, written word is a word with a broader meaning than trigram:

trigram: a word that is written with three letters in an alphabetic writing system
written word: the written form of a word

trigram (noun)	written word (noun)
a word that is written with three letters in an alphabetic writing system	the written form of a word