For first aid purposes, once the victim is safely away from the burn source, call an ambulance! Tell the person the victim's age and sex and how the burn was caused (chemical, electrical, fire, etc.).
Then, elevate the burned area if possible, and cover with cool, moist cloths, like damp paper towels or similar.
Do not try to remove burned clothing from the area, and don't put the victim in cool water. The cloths, elevation and getting the person in an ambulance ASAP are the best, safest things a person can do to help someone with third degree burns. They are serious and the layman just doesn't have the medical training to deal with them safely.