If they are only 3 weeks old, they may lack the capacity to eat solid food.  perhaps u should bottle feed them for 5 weeks.  you can locate the bottles & kitten formula at most pet stores.  good luck!

 You have to try and make the mother feed them for longer. This may mean keeping her shut in the room with them until she lets them feed. If this doesn't work you need to give them carton kitten milk. They should be able to lap but if not use a syringe or dropper. At four weeks you can start with very milky weetabix once a day, building it up after a week once their stomachs have got used to solid food.
Don't bother with kitten food just yet.

 Give them rewards or something once they finish their food. Or, you can just leave the food on the floor where they can eat when they are hungry, because kittens will eventually get hungry! If these suggestions don't work, and yours don't work too, go see the vet.

 You can get milk "formula" for kittens at walmart in the pet isle.

 If they are at the stage where they still need milk, get them some little per nursers at the pet store, ask your vet how much and best kind of milk or formula for kittens. Give them a thickened formula of rice cereal and milk, and during this time continue to offer them cat food at night.