In fact there are 6 degrees of burns, not 3

You may have heard or read in the news about the situations in which the victim received a horrific third-degree burns. Such burns are the most severe of those that you can still try to heal without extreme measures, such as skin grafts, and amputation. Subsequent severity generally are not compatible with life.

4th severity of the burn injury is characterized by not only the skin but also the muscles, ligaments and tendons. Typically, this happens when a strong electric shock. Survive is possible, but the chances are slim. Necessarily require extensive skin grafts.

5th degree - everything burned to the bone. If the whole body is not damaged, but only some kind of course, you can try to save the victim by means of amputation.

6th degree - burned everything, and even the bones, at least, were charred. In this case, death is inevitable. As a rule, 6th degree burns diagnosed during the autopsy.

via factroom.ru