As 3500 calories burnt equals 1 pound of weight loss, so 42,000 calories need to be burnt to lose 12 pounds. So, you got to eat 42,000 calories fewer than you burn during the 3 weeks to lose 12 pounds of body weight. Now for this target to be achieved, there are certain do's and don'ts, which if followed strictly, the target becomes attainable.
Simply you need to rely less on junk, sugary and processed foods and include foods in your diet, that are high in proteins and rich in fiber.
You can also go by the 40/40/20 rule which suggests you to consume 40 percent healthy carbohydrates, 40 percent proteins, and 20 percent healthy fats at every meal.
Furthermore, try to avoid sugary beverages like milkshakes and sodas for 3 weeks and drink plenty of water, as well as coffee and green tea. Coffee has been known to boost the metabolism, so it is recommended an hour before exercise, to feel less tired. Green tea is the healthiest beverage, especially for those aiming for weight loss as it aids in fat burning. You can buy a healthy green tea weight loss supplement from this list of famous weight loss supplements.
Additionally, exercise, if not strenuous workouts, then simply some sort of physical activity will do. Try doing some pushups and lunges which target the upper and lower body respectively. Go on a hike, scrub the floor, take the stairs when you get a chance, rake the leaves, take your dog out or do the laundry. Just walking is a great exercise, if done 20-30 minutes a day.
Basically, keep yourself physically active. Maintain your muscles too because this increases the metabolic rate which is an important factor in burning calories, even at rest or while you are sleeping.
Summing up, follow the instructions listed above eat a nutritionally balanced diet and perform some workouts consistently to obtain the desired results within 3 weeks.