Three days, 200 miles, does not require a particularly special diet. Just normal food, with a quick energy boost as needed. You can get the quick energy from Snicker bars. Ride slow and you'll burn fat and not need quick enery.
Hydration is more important. Drink more water than you think you need, and if hot, a btl of V-8/day will replace lost minerals and give you a good dose of vegetables.
FWIW, my staples are oatmeal, bagels, peanut butter, hard cheese, M&M trail mix, cookies, pasta mixes, tuna/chicken, and Snicker bars.
If you count calories, figure on about 3500 per 50 mile day.