I am a mother of two kids. I would definitely take your baby into the doctor. A 3 week old baby could get a virus such as RSV which can be fatal to an infant. A newborn baby breaths mainly out of the nose and when that is clogged with mucus it can cause them to go into respiratory distress. It is not until 6 months old until a baby will start to breath out of the mouth and the nose. I know this because my first born was born at 3 pounds and spent 1 month in the NICU. Don't wait and don't feel bad about calling your doctor. Better be safe than sorry. Let me know what happens.