I would go 25 sf for the run, that is what Storey's Guide recommends if the ducks don't have access to pasture during the day, so I would err on the side of roominess.
Hillside is okay if not too steep. What is the slope?
Text is an awful way to communicate, so please know my next question is meant kindly and gently. When you wrote "green roll out stuff," were you talking plastic? My answer to that is an emphatic NO.
You may not think you have predators now. They will hear and smell the ducks, and they will come for them. I am very sensitive about this topic so bear with me. Take a look at this
https://www.backyardchickens.com/t/...-it-is-and-what-it-is-not-duck-security-issue
We get some severe weather here, too, and a larger coop will not make it warmer. I built a double-walled, insulated shelter with over a foot of bedding and it was not warm enough for my runners. They are now in a walkout basement pen where it gets no colder than 40F. Each flock is different. Since you only plan on three, you will need to watch carefully. Thermometers, especially min-max thermometers, are a great tool for managing the flock. It will track how cold the inside of the shelter gets at night and you can gauge the ducks' reactions. There is a difference, too, between surviving and thriving.
If you make a shelter that is about four feet tall you can put up to two feet of dry bedding in it and that will help a great deal. Either double-walled with something like perlite or vermiculite for insulation (it cannot mold or catch fire), and that will be pretty cozy. Something that needs to be balanced with housing is insulation and ventilation. Ducks need a well ventilated space, or you will have potentially fatal problems with mold and moisture. Ducks can get pneumonia.
They are also sensitive to ammonia, so minding the bedding is important. I have a porch area so that water is not in the shelter. That keeps the bedding dry. Sweet PDZ or dry peat moss can help prevent ammonia formation, as keeping the bedding fluffed.
Fence height of three feet is fine for those ducks. But a secure area will have fence across the top. So the next question is, what fence height will be comfortable for you to do the pen cleaning? Mine is about a meter - just over three feet. But I am short and flexible. I did make a hatch door so that I can stand up and reach most of the pen area from that spot.
Foxes jump, raccoons and other animals climb.
Our Day Pen is 10'x16', with coated chain link all across the bottom, secured between to 6 inch wide boards around the perimeter of the fence. The sides are 2"x3" coated wire, with a strip of 2 ft tall coated 1" chicken wire along the sides. The top is covered with the 2"x3" coated wire also. The night shelter we used at first had half inch hardware cloth over every opening, top bottom and sides, what is not plywood or lumber is hardware cloth. All doors and gates have keyed locks. I have two strands of equine electric fence around the night shelter.
