Unlike conventional 2D charts, 3D charts have distinct walls and a floor. Also, by default, these walls and floor show no fill and not even a border. The lines you see within the walls are Gridlines, and not borders. If your slide background is a solid color, these defaults could work best for you — however you may want to play a bit with changing the fill or line attributes for the wall and floor elements. Do remember though that you should leave these elements in their default state unless you have a compelling reason to change them!