Originally Posted by seypat

As both a runner and a cyclist, you really can't compare the 2. Maybe if you were strictly riding a single speed. Take the gears away and it becomes a lot harder time/effort wise. The type of terrain on the route is so important. Same for the individual doing the efforts.

Since this will eventually become a pissing contest between running and cycling, here's what I think:

Aerobically, swimming is way harder than the other 2 for me. You have your breathing limited(compared to the other 2) and you're flailing away with all of your body. It's low impact, but requires more effort than the other 2.