written word vs trigram

written word and trigram both are nouns.

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

As nouns, trigram is a hyponym of written word; that is, trigram is a word with a more specific, narrower meaning than written word:

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

Other hyponyms of written word include bigram, tetragram.

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