Ask your teacher how the grades are weighted. If the midterm counts for significantly more, then you might still have a lower grade. If those are the only three tests and they are weighted equally, you should have an 80 in the class. (Or a 79.66666666etc.) Make the teacher give you a written description of how s/he is figuring out your grade. Keep your tests. If you can’t keep them, take a picture of the front page.