I wonder how things would go if you took the most aggressive duck and put her by herself. That will give the scared one time to bond with the middle ground duck. I'm not sure but let's see what @Miss Lydia thinks.
A couple more knowledgeable and helpful people are @Isadora and @Quatie
I believe...
I'm not trying to discount Miss Lydia's words of wisdom by any means. She is very knowledgeable and has helped me on numerous occasions. I also have a duck that needs extra calcium occasionally, in spite of her eating oyster shells. It must be a fine line with some birds in having the perfect...
You might want to think about offering them food that you don't add calcium to. Just leave oyster shell out available to them for whenever they need it. Just as a bird having issues with too little calcium, too much calcium can also be problematic and cause soft/shelless eggs. Do you mind going...