Three weeks is quite a staggering claim. The human body can do some strange things to stay alive. I am not sure about an ordinary joe being able to be cold for that long and been OK with it. There are serveral Farkis and Buddhist monks that have meditated for phenomenal periods without water, some of these must be genuine. I do agree with Jodie about the breatharians sound about geniune as an e-mail wanting your bank account details so a nigerian prince can cash a cheque.
Anaestethologists can kind of induce a hibernation state in people. In russia where ice is easier to come by than heart-lung machines it is common practice to chill up heart patients before surgery.
http://www.upenn.edu/pennnews/current/1998/022698/Doctors.html
I have also read of case where a baby born at 23 weeks was declared dead, left in corridor for several hours. On been wheeled to the morgue the porter heard a faint noise. The baby was very hypothermic, but survived. The wierdist thing about the case is that baby is now a normal healthy child. Under present medical practice 23 weeks is the edge of life. Of the few that survive the vast majority have developmental problems. As with the hypothermic anaestethia the cooling of the brain may of had a protective effect against bleeding.