If you want to get a flat stomach in 3 weeks you're going to need to take some serious steps. There is no time to waste and there must be no excuses to make since you need to be focused on your goal to have any chance of achieving it.
In order to get a flat stomach in 3 weeks you need to do 3 things:
1. Perfect your diet - What you eat will either help you burn more fat or it will make it harder. You need to be sure that you're not filling yourself up with useless and emptycalories as these will quickly convert to fat.
I recommend eliminating sweets, alcohol, refined sugar, fast food, fried dishes, chocolate, chips, and other salty or sugary snacks, as well as cutting out sugary sodas. All these just pile up the empty calories within you, making it very hard to achieve a fast fat burning. Unless you're able to burn fat, you will never be able to get a flat belly, in 3 weeks or ever.
2. You have to make every workout count. I mainly refer to the strength portion of your workout. You need to focus on exercises that stimulate multiple muscle groups such as squats, lunges, dumbbell swings, pull ups, push ups, chin
While ab exercises should be a part of your workout, I recommend not focusing on them as they're fat burning contribution is limited. To get a flat belly in 3 weeks, you have to focus on fat burning and nothing more.
3. You should also incorporate cardio workouts into your routine to accelerate the fat burning process. I recommend intensive workouts such as running, rowing, kickboxing, spinning classes, and aerobic classes. You can also do outdoor cardio which is also something I highly recommend.
If you're looking for an abs program that can help you make sure you make all the right moves, I recommend the Truth About Six Pack Abs program as it's the best program for people who wish to lose stomach fat and get flat abs.
You should realize that unless you make a real effort you won't get optimal results in 3 weeks. It just won't happen. I wish you the best of luck.