three-decker vs war vessel

three-decker and war vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
three-decker	Yes	No	No	No
war vessel	Yes	No	No	No

As nouns, war vessel is a hypernym of three-decker; that is, war vessel is a word with a broader meaning than three-decker:

Other hypernyms of three-decker include combat ship, warship.

three-decker (noun)	war vessel (noun)
a warship carrying guns on three decks	a government ship that is available for waging war
any ship having three decks
made with three slices of usually toasted bread