Couldn't I technically get free plates and cups using these coupons?
The receipt reflects the coupon. If you take 6 plates back, they'll give you credit for three (since you got 3 from the b1g1 offer).
Plus, they usually won't double count coupons in the first place. If you used a "b1g1" coupon on an item, you can't use a second "b1g1" coupon on that same item. You'd have to pay for a second item to use a second coupon. So the cups wouldn't be free unless you bought even more plates.
In the big picture, though, what's more important--free plates, or your personal sense of ethics? Don't let yourself be tempted in these little things. It's not worth it in the long run. (sorry, had to say it.)