Sounds like she may be sick to me.
3 weeks is plenty of time to acclimate to a new home.
How old is she....and where did you get her?
Do you have other chickens and is she near them?
If a bird is not eating/drinking/pooping and moving around most the time....
...there is likely something wrong with her.
This is what I do when I have a bird that is acting 'off':
I isolate bird in a wire cage within the coop for a day or two....so I can closely monitor:
-their intake of food and water,
-crop function(checking at night and in morning before providing more feed),
-and their poops.
Feel their abdomen, from below vent to between legs, for squishy or hard swelling.
Check for external parasites or any other abnormalities.
Best to put crate right in coop or run so bird is still 'with' the flock.
I like to use a
fold-able wire dog crate (24"L x 18"W x 21"H) with smaller mesh(1x2) on bottom of crate under tray.
Then you can put tray
underneath crate to better observe droppings without it being stepped in. If smaller mesh is carefully installed, tray can still be used inside crate.