If Int'l First is profitable enough for certain routes, then an airline will want a fleet that can always provide Int'l First for those routes, which means that they will have extra planes with the First Class cabin. On any given day, some of those planes will be flying routes where Int'l First is not profitable. I think that an airline like United might retain a certain number of three-class planes even if they cut back on Int'l First overall so they can continue to milk the routes where Int'l First pays and cut their losses on others.