Running plus cycling is quite a feat already, if you do it serious. I'm a cyclist and 15 months into running I still dislike it but also know it's a good thing for me.
I was a runner first, but years made my knee joints brittle, and biking is amazing to avoid pain there. And talking about knee joints, single leg squats are magic to restore them a bit. I was quite surprised.
Why not swim.
I think about it regularly (cause swimming is one of the best kind of exercise I know of too) but it's less convenient logistically and also psychologically, you can't swim alone unless you have access to the sea or a clean calm river.
Yeah good point about solitary swimming.
I was in a permanent state of back pain when my only exercise was cycling/running. Adding a bit of resistance training is what finally cured it.
Carefull though. I busted my knees combining intense ice skating (multiple hours per day) and explosive running/jumping (basketball). Two very distict movements for that area.