Which Discipline For Weight Loss

running works best for me

I agree running seems to work the best for weight loss.  But the nice thing about tri is you don't have to run every day.  My legs just can't take the pounding, but  can still do something just about everyday with swimming or cycling.

That is one of the reasons many folks get into tri, it is natural cross training.

Doing something every day is better for weight loss than just running 3X a week.

Your body will get used to the pounding as well just watch how quick you increase your mileage. 

All of them work.  Biking because you can go for a long time.  Running because its the highest HR/exersion. 

But don't underestimate swimming.  Yes, you don't get quite as high of HR.  But your body burns a lot of energy keeping warm.  There's a reason you're often ravenous after swimming and thats because you've burned a lot of energy to stay warm.  You lose heat 25 times faster in water than air. 

Riding a lot has always made weight loss easier for me.  For toning, a consistent combo of all three does the trick.  Riding makes my legs look great, running improves my core, and swimming is great for the upper body.

I think that combining the three is the best bet, especially for someone with joint issues.  As mentioned above, you can keep your volume of cardio activity higher as biking and swimming give your joints a break from the running.  Swimming may not be the highest calorie burner, but you can also jump in and do a swim when your lower body needs a recovery day instead of missing a chance to burn any calories at all.

The older we get, the more diet dictates body fat.  Without a good diet, it may be difficult to achieve your desired body comp.

Lastly, why not toss in some weight training?  Many people find it to be a necessary component for fat loss along with the cardio training and healthy diet.