Well, the brain would initially use glucose: that would be made from protein by a process of gluconeogensis. That could come from your muscles, specially if you were starving yourself. Obviously, that's not recommended.However; you have to starve yourself for 3 days? I've also discovered that your body needs to break down some muscle tissue in order to produce enough ketone bodies to fuel your brain. Any thoughts on this?
The usual route would rather be a high-fat and very low-carb diet with adequate protein to maintain muscle-mass. But it takes some time. Whether it's needful for anyone who doesn't have some special medical problem, such as epilepsy is another matter ...
I tried to find something online on keto-adaptation. This looks like it would relevant, but I haven't heard it yet:
Steve Phinney and Richard Johnson: Ketones, Uric Acid, High Fat and Health | Me and My Diabetes