The Model 3 manual says not to use seat covers. BUT I have a hairy dog as I am sure many of us do. I would appreciate any suggestions--short of not allowing Janie in the car's back seat.
Check with Tesla, but I believe the manual is referring only to the front seats as having the side air bags. There may be no such restriction for the back seat. As a work-around you can loosely drape a blanket or towel over most of the back seat area by using the two head restraints as anchor points. I don't have a dog, but that's what I use when transporting my cat in a pet carrier (front passenger seat) or the less-than-clean humans and large objects either in the front or the back.
People have reported covering back seats with no issue.
Plug this into google to check for more posts on seat cover for dogs:
site:forums.tesla.com seat cover dog
Use different search terms to find other things also.