The easiest way to lose 3kg in 1 week, is to be clinically obese and eat a 1,200 calorie/day diet and do 2 hours of cardio on an elliptical each day.
If you're not clinically obese and you're not willing to do that much cardio, you're not going to be able to sustain that kind of weight loss and it will be more productive for you to focus on 2lb of weight loss each week(.90kg/week).
Take your weight, multiply it by 15...that is the calories your body needs to maintain it's current weight.
Eat 500 calories less than this each day to lose 1 pound a week.
Add 1 hour of cardio each day to lose an additional pound a week.
To create more of a deficit to lose more than 2 pounds a week, you COULD cut your calories down lower, but you should not go lower than 1,200 as a woman or 1,500 as a man.