In your climate, you should probably have 2 brood boxes for overwintering. It seems that you have that now. You shouldn’t need a third brood box. Please make sure that the following are all true before you add another box or Super:
- Every frame has fully drawn comb on most of the frame, and
- Every frame is 80% full of food or brood, and
- Every frame is completely covered with bees
It is actually a bit late for the Super in most areas, unless you have a strong Fall flow from something special locally. Most nectar flows are over by mid-July. Nothing to lose by adding the Super and see what they do with it, if the above rules are all true. They will take to it faster if you smear some burr comb across the plastic frames.