Is pregnancy normal if had an ultrasound at 3 wks and see no gestational sac?

It is normal not to see a gestational sac on an ultrasound at 3 weeks of pregnancy. In most cases, the gestational sac becomes visible on ultrasound around 4-5 weeks of gestation, which would be a week or two after your positive pregnancy test.

At 3 weeks of pregnancy, the embryo is still very small and can be difficult to detect on ultrasound. The gestational sac, which contains the embryo, yolk sac, and amniotic fluid, may be too small to see at this early stage. Don’t worry, as your pregnancy progresses, the gestational sac and embryo will become larger and easier to visualize on an ultrasound.

If you are concerned about your pregnancy, it's best to talk to your healthcare provider.