I can't find an emmbeddable trailer online, but oddly enough found this:
Slant Magazine, which has a thorough plot description including screen shots, describes the film as "principally about the intersection of two extraordinary fencers: Musashi and Kojiro Sasaki. Everything in the story builds up to their inevitable duel." DVD Talk says the trilogy is "A colorful historical epic that also can serve as an excellent introduction to the jidai-geki ("period drama") genre and postwar Japanese film generally". Criterion Reflections opens by saying, "As the grand finale of an epic, heroic film trilogy, Samurai III: Duel at Ganryu Island delivers admirably on the promise put forth by its predecessors".