If they are moderately large, they require medical attention. Third-degree and worse burns involve charring or whitening of the skin. The burn has penetrated through the entire thickness of skin layers. Having burns debrided hurts like hell, even with drugs, but it can be necessary to prevent infection. Let your doctor make the call on whether grafts or debridement are needed.
Burns are nasty, disfiguring, painful, and far more trouble than they're worth.
If you burn yourself severely enough to require grafting, the surgeons will put you under general anesthesia, take an instrument that's kind of like a carpenter's plane or a cheese slicer, and peel a section of skin from an unburned part of your body to graft onto the damaged site. It hurts, it's boring, and if it gets infected you have to do it all again. And few things in life have hurt me as much as a healing donor site did. You don't wanna deal with it. Once more, with emphasis, if you have a bad second or a third-degree burn, get your ass to an emergency room.