Source : Webster's Revised Unabridged Dictionary (1913)

Three-decker \Three"-deck`er\, n. (Naut.)
   A vessel of war carrying guns on three decks.

Source : WordNet®

three-decker
     n 1: made with three slices of usually toasted bread [syn: {club
          sandwich}, {triple-decker}]
     2: any ship having three decks
     3: a warship carrying guns on three decks