Nak Bin,
like one of the other posters I also did get my licence at EFT in Florida. European licence. There are only a few such schools in the US, and as I am living in Asia this kind of route was perfect for me.
Doing it in 3-4 weeks is possible (I have done it, but only exams and flying). If however you really start from scratch I doubt that studying, learning to fly and passing the exams is possible in 21 days. Possible in 5-6 weeks I reckon, but you got to stay focused as in either case these are no holidays. But what a great time.