Can I beat grade 3 astrocytoma?

Yes. In general, only 25% of people are alive at 5 years with this diagnosis. However, there are many subsets that do better, and we still have not figured out completely why some patients do better than others. I would recommend pursuing treatment because some people do beat grade 3 astrocytoma, and likely treatments will only become more effective in the future.
For a while. Don't expect a cure, but you may have several more good years -- and perhaps there will be a breakthrough. Best wishes.
Astrocytoma. A grade 3 astrocytoma is a serious brain tumor that needs aggressive therapy including chemotherapy and sometimes radiation therapy - I assume you are being treated in a comprehensive center that is able to provide multi-modality therapy for brain tumors as this is key in the treatment.