Yes, loading is shared among A, B, C. The bottom flange, A' is no stronger than before C was added.
Imagine clamping the frame in a vise at about mid-way along one side and then bouncing the opposite side up and down. Wouldn't a rectangular box shape distort more readily than a triangular one? Is this the type of flexing that boxing is aimed at?
Clamp it at one suspension mount point and bounce the frame at the diagonally opposite corner. Would the triangular cross-section resist that flexing more than a rectangular cross-section?