I would take two and see how you handle the work load first. I'm sure you've heard the saying "if at first you don't succeed try, try again." Well we have a similar saying in pre-dental, "If at first you don't succeed it will haunt you for years and no one will ever forget it." Take 2 classes and make sure to do well in them, physics is the least important prereq for dental schools, you can afford to push it off.