It's honestly very possible depending on the setting. I've gone over two days before without really realizing it while camping. Part of it is just not eating that much, part of it I think is some subconscious desire to avoid shitting in the woods or in an outhouse. But I've had it happen multiple times where we go camping on a Friday night and I realize late Sunday that I never went.

I realize that's not the same as three full days, but I have to think if I was intentionally eating very little it would be attainable.