Doctor diagnosed me with chronic Pansinusitis he told me to use 'Rhinocort Aqua 64 micrograms' 12 sprays '768mcg per day' this is 3 times higher than maximum recommended dose 'according to company is 256mcg' is it safe to exceed recommended dose?

I wouldn't do it. With pan-sinusitis it is important that you have been evaluated for aspirgillosis, a common fungal cause of such an issue & must be treated specifically. Nasal steroids may exacerbate the problem.
Several conditions. Testing for the cause is important in the evaluation. Have you had testing to determine if you have allergies? aspirin sensitivity for example?
I also would recommend speaking to your physician about the dose, and whether nasal saline is useful.