Some of the other answers here seem to miss the fact that you would use three of the A brackets in each corner to tie the three panels together.
I am going to make the assumption that you are going to use nutplates & bolts to assemble the box, with through holes in the aluminum plates. I am also going to assume that the bolt head will be on the outside of the box and the nutplate affixed to the bracket on the inside, because how will you tighten the bolt head from the inside for the 6th side?
Taking all this into account, I think A is your best option for strength. However, it will take more care in mounting the brackets than the other two options to ensure the corners line up properly. You'll want to space them as closely together as possible without interference. In fact, I'd probably discourage this method unless you are trying to use your box to lift a car.
I think a modified version of B is your best overall option for simplicity and ease of installation - I would go with something like this:
This will allow you to put the nut on the inside (or possibly better yet, tap the holes to match your bolt thread and screw your bolt directly to the bracket itself, eliminating the need for nutplates).
In either case, you could add additional A brackets along the sides of the box for extra strength.