That’s the hardest part of having an older larger duckling in with tiny ones. First off they need a larger brooder then you can have all 3 together but make a separation using something they can see through to see each other an share the heat lamp. I used a piece of hardware cloth wrapped in gorilla tape and use it as my separation when I had new littles an 3 weeks old in the same brooder. Hopefully the little will recover and just got a slight sprain. Just make sure all 3 are getting the b complex daily for leg support.