Hi, kidsnchicks,
I pretty much follow Storey's Guide on space, and my eleven runners seem healthy and happy. I have a 32 sf plywood double-walled box with sloping transparent roof, a sliding duck door and a Dutch door on the side for me. It opens into a 32 sf "veranda" wrapped on all sides with half inch hardware cloth. This is their night area (we have many predators in the neighborhood). Storey's recommends minimum 3 sf per duck for their coop/house, more if they are a large breed.
For a run, Storey's recommends 10 sf per duck if they are let out to free range, 25 sf if they are confined all the time. I have 160 sf for their day yard (also predator resistant construction) and let them work the gardens some days.
The house, veranda and day yard are all connected so that they have use of about 224 sf of area during the day. They have two concrete mixing pans for swimming, two very large steel mixing bowls for additional drinking water, a large cake pan for pelleted feed, a dog bowl with oyster shell in the bottom.
Hope that gives you some ideas for space.
Added:
One female for a drake may not be enough for her sake. I am told drakes need three or more females or they can hurt the female with too much mating (also reduces her laying). If your other two are female, that might work.
My ducks have used the nest boxes I supply for them. One or two use them regularly. They prefer to make their own. They have wood shavings with a topping of straw for bedding, and they can make nice nests from that. Since they are not free-ranging, I don't have to go on egg hunts as a friend of mine does. Even though mine now make their own, I think a simple box with a wooden egg might get them started.