If the puppies are not crying they are getting enough to eat. It is natural for her to start moving away from them a little more at 3 weeks. You probbly saw her almost not wanting to move away from the pups at all for the first 2 weeks. I actually had to physiclaly pick mine up and take her outside for potty breaks as she did not seem to want to leave them at all those first 2 weeks. But by the 3rd week they venture away a little more and do not nurse non-stop. That is fine. Next week you will get to start adding some puppy food and that will give momma even more break time.
Novice ex-breeder with lots of research and only 2 yorkie litters.