3 grams of protein per potato isn't much. If you're using this diet to lose weight you'll be eating say 1500 kcal per day of potatoes (leaving some allowance for oils), which will only give you ~39g of protein.
That's definitely below what most people need as a healthy minimum (RDA is 0.8g per kg of bodyweight), let alone if you want to build muscle while dieting which is a good idea for improving health. At the very least you want to maintain muscle mass, which requires protein if you're exercising.
If I was going to try this diet I would definitely supplement some protein shakes.
