In my experience domestic ducks are not good mothers. I suspect this is because there tends to be a lot of egg incubator use in domestic duck breeding. Darwin does not weed out the poor mothers so, they get more prominent.
Ducks also tend to lay fewer eggs than chickens.
When the duck hen is laying eggs, keep them (as you have 3 I would keep them all together all the time) in a smaller area that you can check easily. Gather the duck eggs and put them under chicken to hatch. You will want to put them all under the chicken hen at the same time without any other eggs. You may need to use additional 'fake' eggs to satisfy the chickens egg sitting needs.
There is a bunch of complexities in the above that I have not fully addressed. Maybe we need more related questions?