They don't eat yet. They suckle. They are too young to be weaned and would be drinking their mother's milk. If they are orphaned, they would have to be bottle-fed. Kitten "formula" can be purchased at vet and pet stores.
Usually kittens start eating small bugs and cockroaches when they could catch them with their paws. Some of mine were even interested in eating mice. Better be sure that the food is neat nd without contamination.
Three week old kittens need their mothers milk. They can not eat dry food. If the mother is not avaliable you can purchase formula from your local pet store. Do not feed them cows milk. They can not digest it and it does not have the nutrients they need.