You've been given great advice thus far. If your kitten is, in fact, only 3 weeks old, she is far too young to be away from her mother and will require constant care. As mentioned, she still may need to be stimulated to potty as would normally be done by her Mommy cat. Not urinating or pooping will become fatal. No, it's not at all normal for a cat not to potty for two days. Please take her to the Vet for a thorough exam, to confirm her age, and learn more about the type of care she needs.
With the proper care, kittens this age often thrive wonderfully. My Sierra was just a few days old when we met, and Serenity was almost 3 weeks old, just like your little one.
Congratulations on your new member of your family and thank you for wanting the best for her. She's a precious baby and completely depending on you!