What would happen if I used frontline on my 3 week old kitten

Frontline's packaging says to not use on a kitten younger than 8 weeks old.

Rather bathe the kitten in very little warm water & Dawn dish soap, comb through it's fur and try to pick out the fleas.

Fox is so right here. We asked our vet that once and she said the same thing Fox did. Dawn soap, warm water, and picking out any found fleas.
It can cause all sorts of health problems hun to use a flea med on such a small kitten. Also, if the mother has frontline on, then some will rub off on kittens (slao a quote from our vet) and that can be OK as it is not a dirrect application.