3rd day on amoxicillin 500 mg every 8 hours prescribed by my dentist before extraction. still don't feel any better. i used this before for tooth infection before extractions and i felt better, not the case now. is it too early or is it not working?

It may be working, but slowly depending on the extent of the infection and cause. Antibiotics are carried to the infected site via your bloodstream... if there is a big pocket of pus there may not be blood flow in the center. The infection may need to be surgically drained. Call your dentist for guidance.

It could be both but I would give it at least another day. Minimum of 48 hours. It could be a resistant bacteria but the main aim of antibiotics is to reduce the infection to make the surgery and anesthesia easier for you.