don't forget that wavefront errors propagate. A threefold two mirror system is effectively a three mirror system when you add a mirror diagonal. Are GSO flats 1/10wave p-p or rms? If they are p-p rms then 3 refelctions will give you a 1/3 wave system, if they are rms, you'll have more like a 2/3 to 1 wave system. Sorry, long day at work & too stuffed to do the exact calcs. If you use GSO's you'll end up with a very very low power telescope.
http://www.nova-optical.com/Flat%20Mirrors.htm
expensive but good.
Joe