IF your training for 3 hours then you simply will have to consume a source of nutrition during your workout, post workout you will want to eat starchy carbs such as potatoes and pasta. you also want 50-70g of protein. so an ideal post workout meal should be eaten an hour before and could be something like rice with new potatoes and fish or chicken. If your cardio routine is hard then perhaps you could take some sports drink such as gatorade, but try to stay away from sports drinks because they can weaken your bones, only use them in strenuous workouts. other wise pile the water down your so you are fully hydrated, when you hydrated your up to 33% percent stronger. during you workout also eat a bannana or you could take a shske with carbs in it, these will provide you with potassium and electrolite carbs which fuel your energy. within about 30-40 minutes of finishing your workout, eat your main meal with your largest source of protein for the day, you can eat alot of of protein in this meal around 80g in order to fuel the repair of you muscles! hope that helps!!