For how long flixonase nasal spray can be used safely? is 3 months too long?

Depends: on the dosage, frequency, indication and response, also if you are using it right. If you used it that long you need to be supervised by a physician / ENT specialist, true that flixonase (fluticasone) is safer than other steroid nasal sprays, but needs to be monitored and rightfully prescribed, yes some patients use it that long, but again need to be medically supervised, good luck