Could a triplane fly using a dihedral wing in the middle? I wonder if a triplane glider whose front view is shown in the figure can be built and flown successfully.  As you can see, the middle wing has a dihedral angle, but the top and bottom wings do not.  The vertical lines represent structural members.  The idea behind this strange design is that a box-shape with diagonal members is structurally very strong, hence my interest.  I realize that the structural members at each end will contribute to the drag, even if they have a streamlined shape.  I also realize that every corner of this shape will not produce much lift, but will produce much interference drag.  Yet, I wonder if this weird triplane will fly properly at low speeds.  Any thoughts?
It would depend on the relationship between span and separation. The dihedral would be too great on a short-span wing, as drawn, leading to all sorts of issues. Since the separation needs to be more than usual, approaching two chord lengths, you would need a very high aspect ratio, maybe 15 or so.

But if you were to leave out the middle wing and cross-brace each side with thin wires, as a biplane it would be just as stiff and perform a heck of a lot better.