Aim for that loss in ~8weeks. Then doeable. If you try some crash diet
course then you will gain 10 kgs when back to normal food. There are ways like water fasting, juice fasting etc but that's more for detoxifying body than wt control.
Be realistic. calculate:
1lb fat = 3500 calories.
3kg would be roughly 24000 calories and in 6 days it would mean 4000 calories/day. Is it possible? Perhaps yes ... water fasting but even that is doubtful.
back in school days we were made to drink vinegar, dress up in all jackets,multiple pullovers and then told to do 12km run on hilly track (boarding school days) but even that didn't burn out so much s fast.