To lose around 15-20lbs / 6-10kgs in 3 weeks? (at least 6kgs)
I don't really care too much about it not being *completelyhealthy* but is it possible?
I have about 11kgs to loose before I become considered underweight, that's according to my BMI...
Also can someone tell me just roughly how much/what exercise I should be doing each day?
Thanks in advance:)

About the only way would be with something like slim fast, and I can't guarantee how much or how fast, you'd have to look into it, but their product is nutritional healthy and has what you need daily requirement wise, Exercise wise, 30-60 minutes a day, 4-5 times a weight of all body workout exercises, like at least 20 minutes of cardio, and then resistance training, or weights, walking, jogging, 1 mile, biking or swimming, dancing, you need to keep the body moving, hard for at least 30 minutes each session. Also drink lots of Ice Cold water, as much as you can handle through out the day, Drink 1/2 your weight in water, (divide your weight x 2 for instance you weigh 140 lbs that = 70 oz. or about 9 cups of water) eat protein with every meal, 1 gram of protein per pound of body weight. Lots of veggies and fruits, if you try slim fast stick to exactly what they tell you to do, they have the best and safest, quick results I seen.