How to lose weight?? Im 3 stone overweight....?

So im not HUGE but I am overweight.. I do lots of exercise and I have seen people the same weight as me and they are HUGE so lots of it is muscel im thinking.. (when I say lots of exercise - I mean lots, im on the swim team and train 1hr+ everyday of sprint crawl) ALSO im female... im thinking its the coffee. I have 3 mochas a day...