I think Marciano would beat both of them, Langford by ko and Dempsey by late stoppage or points. After that, I think Dempsey ko's Langford. All of them would give each other a hell of a fight, but imo Marciano>Dempsey>Langford.
Please to remember that Langford twice stopped Wills, a result that is arguably better than any one or two results accomplished by Dempsey or Marciano. He must be considered seriously in this tourney.