My three year old sleep quite well in the afternoon at home, but does not sleep in the day care, perhaps due to so many kids being around. How can I help her sleep again?

Well! It is obvious to happen. You know how it is if we have to sleep at a friend's place or somewhere else! We have a sound sleep at a place where we everyday sleep, so same is with the baby. She would take some time to adjust, and will adapt. May be you can tell the daycare person to spend a little more time patting her, or you can let her take her favourite toy, and she can sleep while keeping it by her side. Do not worry, she will soon adjust.