Which of these will strongest connect three panels?
B seems like the obvious choice.

You can easily rule out A because it will require two brackets, minimum, and even with those two, one of the seams will be weak.  To make that seam strong, would require a third bracket, and it is generally undesirable to increase part count when you can achieve similar results with a smaller part count.  (But I'll admit you didn't say that was part of your criteria).

C has the same problem.   Two of the panels will be tightly connected to one of the panels, but one of the seams is weak and can flex open more than the other two.

The obvious advantage to B is that each panel will be directly tied to the other two panels, which means every seam will be much tougher to separate (flex open) than A and C.

Another thought:  B in the corners with A at the midpoint of the panels would be a really nice tight box.