Juice the mangos, peach & lime(make sure if you are using frozen peaches or mangos, that you allow them to thaw to room temperature for the most amount of juice). Add agave nectar or honey and place in a saucepan over medium high heat. In a small bowl, combine cornstarch and water until combined. Slowly stir cornstarch mixture into juice and bring just to a boil, stirring often. Remove from heat. Mixture will continue to thicken as it cools.