Can you post a pic of the set up? Does she have hides on both ends? Is the cage in a high traffic area? (is it a tub that's clear or opaque?)

Has she ever eaten for you? Were you told she was feeding on mice when you bought her? Live mice or FT mice? Is it humid enough for her? What are you measuring temp and humidity with? How much are you handling her?

There's a lot of tips and trick to try to get BPs to eat, they can be picky little fellows. With some more info I'm sure we can suggest a few tricks to try.

Some of the most common are leaving her alone(except to change water/clean up poo) and making certain that she feels secure by covering three of the four sides(if it's a clear sided cage like a tank or clear tub), putting in hide spots(if she doesn't have some already), trying smaller prey, heating FT prey to a hotter temp with a hair dryer...

3 weeks should not be a cause for alarm yet. I'm certain with some patience and some tweaks to her set up, you'll have her feeding in no time.