All three are different forms of exercise with different demands on muscular/skeletal system. I do not believe one is superior than the other in terms of taxing your heart. I have ran as part of a physical fitness test for my job for over 28 years. I have ran 12 miles in boots and full combat equipment to include a 35lb rucksack in 1:38. I do not experience any difference in cardiovascular demand between running and cycling at high intensity. I can pace myself running just like I can pace myself on a bike. I can run at a sprint and sprint on the bike until my lips start tingling and get tunnel vision and am about to pass out (unless I tear a hamstring running). The difference is I go faster and travel longer distances on the bike. As a result of the different forms of exercise, muscle groups are taxed differently. Which goes back to my original question...how are we defining "harder".