I'd say you have a little Blue Swedish duckling too. Don't see any Runners in there.
I know we're not on anything but breed but that feeder is way too small for those bills.
When they are standing do they stand up tall like a Runner? the one looks like maybe a Khaki Campbell [the brown one with the pretty markings coming in] The Black one maybe a Cayuga? If you can get pictures of them standing that would be great. The blue could be blue Swedish hard to say But they...
I think it’s a good idea to have 2 brooders ducks are so messy plus putting them on their own feed may help. Usually once you get them on the BComplex it doesn’t take long to see improvement 3 days to a week if it’s a niacin deficiency depends on how long they had it before starting on the...
How old is Whiskey now? I am sorry she is still having problems i hope the liquid B complex will help.
Here is info on how to give liquids
https://www.backyardchickens.com/articles/safely-administering-oral-medications-to-all-poultry-and-waterfowl.73335/
Bless it. Be sure that none of the other birds begin to bully the little one. Sometimes that can happen when one is showing signs of having a problem.
This will give you doses and info on Niacin deficiency and becasue it's so young you going to want to mush the mealworms a little so they are...
Make sure it's nutritional yeast and not baking yeast. Big difference. But when one is already has a problem liquid B complex is better.
https://www.backyardchickens.com/threads/feeding-ducklings-medicated-feed-the-myths-and-the-truth.1247838/
Info on liquid B complex and dose for age of...