Swelling three weeks after nose job can be very common. Remember that every patient has his own propensity to swell up in different way and some people swell up more than others. I think that the most useful thing for you at this point would be to massage your nose in an upward direction by gently pinching it to between your first three fingers and moving your hand from tip to the bridge so that you alleviate the swelling. You can do this for 10 min, 3-4 times a day for a month.
At three weeks after a rhinoplasty the nose is often very swollen. This is common and you have to give it time to heal.