You will see all kinds of clothing at dinner on your cruise. Although, on a 3-night cruise I don't think there is really a "formal" night. DH and I have been on 2 7-night Disney cruises and we did take formal (dark suit for DH, dressy pants outfit and a long dress for me) clothes on those. But our last cruise was a 4-night Wonder and we did not take formal clothes. DH wore khaki slacks and either short-sleeve button up, or Polo shirts to dinner and I wore either a sundress or slacks and blouse. We felt fine. Many others were dressed in the same manner. We actually saw very few suits and long dresses on the Wonder.