Unfortunately no, for a 3 week old kitten you definitely need to be try and get it to take milk. If there's no mother then make a formula, of milk, pedialyte, a little and egg yolks, warm it up and the use a nursing bottle.
It is not advisable to feed the kittens with soft cat food.They still fed by the mother cat. Naturally the kittens will not accept the food.If the kittens are deserted by the mother,then this food be given as a substitute.The kittens always prefere only milk.
Kittens are meant to nurse from their mothers weeks, but if a young kitten shows interest in eating soft cat food while it is still nursing it should be fine.
I think you can feed them as long as they want it and are fed up of milk add a comment