I am having dry cough from 3 month, now P.N.S xray showed that mucosal thickening noted in maxillary sinus and frontal sinus appear hypoplastic. Help?

Maybe. People with chronic sinusitis often have a persisting cough as one of their main, or only, symptoms. The mucosal thickening of the maxillary sinus is often indicative of chronic sinusitis. The hypo plastic frontal sinus just didn't develop fully and likely has nothing to do with your cough. I would recommend consultation with ENT physician. Medications and further imaging may be necessary.