I have 2-3 days in Bangkok to extend my Chinese visa for 30 days. Is this possible?

Yes, it is possible for you to get a visa within 2-3 days in Thailand. If you will stay with your friend in Beijing, you need to provide an invitation letter from her, together with her address, copies of her passport information page, China visa or temporary residence permit, or copy of her ID if she is a Chinese. So, if not, hotel booking for few days is ok. Return ticket may also needed.