3 weeks after Rhinoplasty, what precautions should I take when washing my face?

At 3 weeks the nasal bones should be fairly firm and you can comfortably wash your face with hot/warm/cold water without any concern. Always check with your surgeon before you start pushing firmly on the nose in the first 6 weeks. Gentle wiping and massage would not cause any problems.

You should wash your face normally as you would with soap and water.  The temperature of the water does not matter as long as it feels comfortable to you.  

You should be able to wash totally normally at this point. I would not put too much pressure on the nose just because it will hurt but it is unlikely that you will cause any damage by just washing. I would ask your surgeon for more specifics if you need them. Best of luck