Is it ok for my employer to cut or deny me hours due to pregnancy issues?

The issue is whether, in addition to a doctor's note, you had used paid time off (e.g. sick days) you had earned for the 3-day absence, or at least used some employer provided and approved "call out" process for being out for medical reasons (and fully complied with the policy).
If you did--if you used sick days you earned or complied with a call out policy IF your employer has one--then your absences were authorized. If authorized, the employer may not cut your hours or take other action(s) against you because you are pregnant. Discriminating against  a woman due to her pregnancy is seen as sex-based discrimination (since only women get pregnant, to discriminate against someone due to her pregnancy is to discriminate against her because she is female). In this case, you may have a viable employment discrimination claim and should contact the federal EEOC to discuss fiilng a complaint.
But if you did not have and use sick days (or other paid time off, like vacation days) you had earned, and your employer either does not otherwise have a call out procedure or it does but you did not comply, then your absence was unauthorized: a doctor's note does NOT give you the right to miss work unless you use PTO or call out properly when there is a call out process. The doctor has no authority over your employer. If your absence was unauthorized, you could be terminated for it and therefore your employer could legally cut or reduce or deny your hours.