This is an interesting situation since dropping Chemistry would make you ineligible for an ATAR. Based on this, you should consider your goals beyond the HSC. Are you still interested in pursuing a career in dentistry? If so, a rational course of action would be to keep Chemistry (given that it is treated as assumed knowledge for relevant degrees) and try your best to improve in this subject starting now and as you commence and progress through year 12. The good thing about year 11 is that none of your results count towards your HSC marks or your ATAR, which means that you definitely still have a chance to turn things around in your favour.
At this stage, I believe that it would no longer be possible for you to take Physics, especially since you are approaching year 12. Taking a subject in year 12 inherently requires completion of the subject's year 11 course, which is a condition that has not been met in your case. This makes dropping Chemistry more questionable since you would be completely denying yourself the chance of improving in the subject with the aim of maximising your ATAR.
It is also important to keep in mind that, regardless of your specific study interests beyond the HSC, there are still other ways through which you can study your preferred degree (in the event that you do not achieve a sufficiently high ATAR), including opting for a slightly different/potentially longer study path with the aim of transferring or being admitted into your ideal degree at a later point in time.
I hope this helps!
