A good thought. I've thought of this before, and wood sets, too. But frankly, I'm not sure there's much incentive for WGT to give a discount on 3 wedges, since most folks are going to end up buying them anyway.
I do think, though, that 4 club wedge sets, and 3 club wood sets, and discounts on the last one to complete a set for those who already own the others, might actually be a profit maker. It would encourage the sale of hybrids, and wedges that normally don't fit in to a players bag configuration.