Third tray of clear correct not fitting my back teeth. Should I be concerned?

Based on your history, I would suggest contacting your dentist to explain your issue.  Usually, if an aligner is moving, then it is not doing the best job that it can do.  However, your doctor would need to know about this because it is possible that the area that has movement is not a crucial area and may not have any negative affect on the final outcome of your teeth.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMD

Based on what you are saying, I suggest visiting your dentist and having your new impressions taken, and new trays fabricated.  If your current trays are ill-fitting, they will not move your teeth into the correct position.  You also need to make sure that when you have trays that fit well, you wear them at all times to allow your teeth to move into the correct position. 

I would suggest trying to revisit the doctor to have him verify if the trays are defective. This happens. And if it is the case, then some new ones need to be fabricated. It is very difficult to get the results that you desire, when there is no tension on the dentition or teeth. So yes, I would revisit your doctor to be sure that you are progressing properly with your clear correct treatment.