Twentieth Century Fox and Marvel Entertainment set Brett Ratner on Friday night to replace Matthew Vaughn as the director of “X3,” the third installment of the “X-Men” franchise. His hiring keeps the film on track for an August start date and a Memorial Day weekend 2006 opening date. A comicbook enthusiast, Ratner was among the directors considered to direct the first “X-Men”; the job went to Bryan Singer. Ratner then spent a full year developing “Superman Returns,” the film that ultimately prompted Singer to drop out of directing “X3.” Fox’s hiring of Ratner also means that “Rush Hour 3” will most certainly miss New Line’s projected fall start and summer 2006 release dates. Ratner directed the first two movies in that series and seemed certain to make the third, but Chris Tucker’s unwillingness to sign his $20 million deal in timely fashion scotched those plans.