I think that three weeks old is too young to free-range on their own in your backyard, They should remain in the penned area with sun shades. At three weeks old, they still need to be supervised in the wading pool as they can get waterlogged. The wading pool should have stones inside to made steps for the ducklings to get out. Ducks are not good on ramps -- any ramp would have to be very shallow, not steep.
My ducks do not go out free-ranging until they are 12 weeks old. They are in the brooder until 4-6 weeks old [depending on numbers of ducklings and how crowded the brooder is] then in the run for another 6-8 weeks. Even introducing new adult ducks, I have them in a run for at least 2 weeks, where they can see my ducks an my ducks can see them, before letting the new ducks free range. I currently have 2 that I am introducing and after both spending time together in the run, I now have one free ranging at a time, so that I always have one I can put back in the coop -- the other wil then follow back to the coop. That way, they learn their way back to the coop and I will not have to go chasing round hte back garden looking for ducks in the evening