If you keep letting them see each other by the time the 3 week olds are close to same size as your lone one they should be use to each other. Patience is the key just as Amiga said. Ducks live in flocks so I'd say eventually they will be a family and flock.Her feathers look good to me,very smooth and sleek.Her wound looks good almost healed.i got the babies a week ago so they could be almost 3 wks old.They all act scared when I put together and I do sit them by each other in totes.the big one has jumped out to see them though

Sounds like she is healthy probably just needs time to adjust. Please keep us updated..
That is great she is enjoying herself outside.