Laughing after rhinoplasty - would that affect anything? Am I allowed to laugh hard after less than 3 weeks after rhinoplasty?

It is highly unlikely to cause any damage to a nose 3 weeks after a rhinoplasty by laughing at a comedy show.  Depending on the specific techniques used during your surgery, your smile might be slightly affected until most of the swelling subside.  In our practice, we give very specific instructions to our rhinplasty patients regarding various routines to assure unimpeded recovery.  Discuss your concerns with your surgeon.  

Hello,There is no obstacle that can prevent you from laughing, unless you have external excisions. Please consult your doctor for the most proper answer. Best regards 

Although you should continue to be careful about direct trauma to the nose such as from athletic activities,  facial movement with laughter is unlikely to cause any problems.  Have fun at the comedy show.