3 months after sinus rhinoplasty surgery. Is it normal to have yellow mucous discharge?

You may have some inflammation or infection.  Keep an eye on the symptoms and if they don't resolve in a few days arrange a review with your surgeon or discuss with your general physician.

It is a common misconception that discolored nasal drainage equates to a bacterial infection. Discolored secretions occur whenever there is inflammation (viral, prolonged allergic, non allergic, bacterial, autoimmune, etc). What is important is how the symptoms progress over time. I would presume that it this is viral and irrigate with saline and manage the other symptoms. If it's still present after 2 weeks may need antibiotics. If the surgery restored the normal sinus drainage pathways most patients after surgery stop having infections