When you enter the last big open area before the fog door there will be a somewhat hidden walkway on your right leading to a cave with a statue blocking your way. Hit that mother with a fragrant branch of yore, and you're home free. Edit: Wait what? There is no third bonfire in The Gutter. There's just the two. The one I described is the second, and puts you seconds from The Rotten's fog gate. Double edit: Are you talking about the primeval bonfire after the boss? It's right after the room where you fight The Rotten. Somewhat hidden, but just look around and you'll see it.