The third wall is at the end. This is the wall that no one is meant to touch. This wall is the Wall of Death, and to be crushed against it brings hopelessness, fear, loss, and anger. One can reach it without dying, but one cannot pass it and live. Those who have gone beyond and returned are not living the same life they were. Think Christ, whose ressurection transcended this wall, or Mohammed, who's ascention into heaven is not a death, but a movement beyond life. Those who go beyond the Wall of Death are no longer recognizable as human, but are seen as divine.
The time you spend between the Walls of Sleep and Death is inverse to the distance between them. The longer you are beyond Sleep, the closer Death looms. Spending time between Birth and Sleep will pull Death back, but at a slower rate than you draw it to you. I'm tempted to believe that the longer you spend close to Birth, the further the Walls of Sleep and Death get, but I imgaine that all these Walls march closer, with only Birth remaining forever where it began.