[Prod]

Will a broody hen adopt 3 week old chicks or will she only look after chicks she has hatched???

[ANHBUC]

If she doesn't have any chicks it might work.  If she can hear them cheeping for a while as they usually bond before they hatch.  Don't let her see them though.  You would have to try and replace one chick at a time with one of the eggs she is sitting on.  Leave her to settle in between putting the chicks in and listen for any aggression.