my question is which one is most optimal for fat loss?

Running is the best, IMO. Biking comes second, but it needs to be at a "worth it" intensity. Swimming is lasssttt.

the one that you burn most caloreis doing,which is down to you.

I think you lose the most weight with biking. You burn more calories / hour running, but you can't maintain that level of effort long enough to equal the number of calories you'd burn cycling. And since I can't swim worth a gosh darn, I wouldn't even include swimming in the same sentence...