How can you prevent kidney stones coming back if I had them three times this year? I exercise and watch what I eat, but they are still coming back.

Need workup. You need a metabolic stone work up including blood tests and a 24-hour urine collection. These will allow your urologist to determine if there is an underlying metabolic problem that may need to be treated with further diet changes or possibly medication. The best advice I can give is drink enough fluid to make more than 2 liters of urine a day. Lemonade is generally good to decrease stone risk.
Water, low salt. Better to get stone work up done. Please check with your doctor and may need to see a nephrologist. General preventive measures are increasing water intake and low salt diet. Stone work up helps to identify additional specific measures.