I still use Nutritional yeast with my flock about once a week I sprinkle it over top of their feed. So until your ducklings are 10-12 weeks old they should get it every day. After that if they aren't having issues and they are getting top-quality feed it's up to you if you want to continue with...
Hopefully, the B3 will kick in quickly. When you feel along the leg does it feel any different than the one that is working normally?
When you put her in water[hopefully nice and warm] what do you mean by complaining? she doesn't like being in water?
It's just the one duckling that is having problems? that one needs a full dose. The others can have it sprinkled overfeed or treat but you want to make sure the one having the problem gets a full dose by itself. If the little one likes peas put it over some. What the duckling doesn't need it...
Nutritional yeast I hope. But when one is already having serious issues The best thing is Liquid B complex TSC carries one it will say injectable but we use it orally. Since she is eating put one ml over a tasty treat just enough treat she get the full dose. Do this daily until completely over...