My period was 3 weeks late, started my period which only lasted 1 day. Bleeding seemed normal. Could I be pregnant?

Certainly possible. Check a pregnancy test as it is certainly possible to be pregnant in this situation. If it is negative check again (if no period) in one or two weeks. Finally, it is common to miss periods on occasion with subsequent cycles often coming irregular.
Yes. It's common to have bleeding like you describe early in pregnancy. Since your period was 3 weeks late, it is quite possible you are pregnant. A pregnancy test should be positive by now if you are. Go get one done and if you are pregnant, you should go see a doctor as bleeding in pregnancy always needs to be evaluated.