Is it normal that my 3 week old breast fed baby has not pooped in two days?

Yes: It is very normal for babies to skip days pooping. It is quality, not quantity, that determines constipation in infants. If when she does poop it is soft/mushy/runny, then its ok for her to go days in between poops. Some breast fed babies are just so efficient at digesting the breast milk that they don't make much waste, and therefore, not much poop.