How many chickens do you plan to have in there? Are you sure you need three nests? I tried a tractor one summer, 64 square feet for eight chickens, two 4’ x 8’ sections that I moved separately but attached together. I had to move it a lot, about every two or three days, to keep it from stinking. It took them no time at all to wipe out the grass and weeds and scratch the ground into a muddy mess, especially if it was wet. That kind of tractor has limited carrying capacity.
I’m having trouble envisioning an A-frame six feet wide but only three feet high. I’m not sure what is going on. My tractor was a rectangle with the run only two feet high, which was enough.
One of the problems with an A-frame is that you get squeezed for space on top. The advantage is that it is an extremely stable construction and could be lighter to help you move it. A 32” section across that 6’ high and 6’ wide A-frame doesn’t give you much to work with. How much roost length do you really need?
Instead of making the nests as high as you can, consider making them as low as you can to give you more room to work with up top. If you can get by with two nests, you will be a lot better off. Then put the roosts as low as you can with them still higher than the nests. With the nests external you should not have a problem with them pooping in the nests from the roosts.
How do you plan to keep the feed dry and keep them from pooping in the feed from the roosts?
To save weight just cover enough of the top to keep them out of a rain and keep the nests dry. In your summers they really don’t need much protection.