Each physician have different guidelines regarding post operative care. Typically, light activity can resume 5 to 7 post surgery. Bruising and most swelling generally resolve itself in the first two weeks. Residual swelling that can take 6 to 12 months to resolve but not noticable to the general public. Consult with your surgeon regarding your case as he or she will know the scope of surgery and the specifics about you.
Taking your examinations 3 weeks after a rhinoplasty is certainly acceptable. No need to cancel your surgery
Although most patients feel quite well about a week after rhinoplasty surgery, it is hard to predict the effect that your surgery will have on your ability to adequately prepare for your exams. Everyone's recovery after rhinoplasty is different. Additionally, everyone prepares differently for exams. Make sure you allow yourself enough time for recovery.