How old can Rottweiler puppies be able to eat puppy food?

I have eight beautiful Rottweiler puppies. They are three weeks old. Their teeth are barely starting to come out. I was wondering what week is the best week to introduce them to puppy food? Please I need an answer fast!!! Please help. If you do know what would be a good and healthy food. Like pedigree,puppy chow, purina, which one?