Tooth sensitive to bite 3 weeks after filling. Getting worse. What next? Crown? Root canal? New dentist?

This is really difficult to know without an x-ray of the tooth.  I assume that that when you put your teeth together, your bite feels normal.  It is not until you put some food between your teeth that it starts to hurt.  If this is the case, the tooth most likely needs an adjustment on or around the filling.  The filling may be in your chewing pathway.  Most of the time, I have a patient chew on wax or a piece of gum on the opposite side while we mark the side that hurt.  This leads to marks that show up only when the patient is chewing.  At that point, we then adjust.  I would assume that your dentist should be able to do this.If the filling was in close proximity to the nerve, then maybe a root canal is necessitated, but hard to say without an x-ray.  Also, if, by chance the tooth was not bonded adequately to the filling, biting sensitivity could occur.  There are a number of reasons why this could be happening, but I would go back to your dentist and explain and see where it goes.

Sounds like may bd fracture in tooth that will require a crown and maybe root canal. I would recommend you go back to dentist again and discuss symptoms. If there is no response then recommend another opinion.