if she had had a fever for 3 weeks she needs to all the Dr ASAP
do you mean she has a fever today and for the last three weeks she has been feeling ill. i would say it was probably morning/all day sickness for the last few weeks and she might just have the flu now. if she has a fever for longer than 48 hours call the doctor. do your best to cool her down but keep her comfortable of course. hope she is feeling better soon.