Should I switch my 3 week old kittens to kitten formula??

Hi! My cat recently just had kittens, they are about 3 weeks old. She had 8 kittens, 3 of them died, and there are 5 now. 3 of the kittens are like huge, the other 2 are very small. The 3 big cats hog all the food. I don’t know what i’m doing and idk when the right time is to maybe start giving them kitten formula? to the ones that aren’t gaining weight from not getting milk. Someone please give me tips thank you.