Flared Nostrils After Rhinoplasty- Is It Swelling?

It makes sense that as the tip projection is reduced during rhinoplasty that the nostril will tend to flare. It may not be swelling and a Weir procedure may be helpful to clean up your result. Wait a good while before deciding.
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If your tip projection was set back you will get some flaring of the nostrils. It's to early to tell how much versus the swelling though. If necessary nostril reduction can be done in 6-9 months as a minor touch up procedure.

While the shape, of the nose, can be altered by post op swelling after a Rhinoplasty, flared nostrils are not an area effected by this swelling IMHO.  This is more likely caused by manipulation of the tip cartilages and is something that you should discuss with your Rhinoplasty Surgeon.  

As you suggest, the flared nostrils may be secondary to swelling just 3 weeks after your rhinoplasty. We don't know what was done during your operation and without pictures, don't know what you look like. Wait several months to see the changes that occur during the healing process.

Three weeks is way too early to evaluate your results.  Swelling can persist for a few months so there is no need for panic.  Give the results a chance to mature and plan on final evaluation after about 8 weeks with any needed revisions deferred for at least 6 months.