Like DesiDentist said, it's totally do-able. It may be hard to take 3 sciences class in the same quarter, but you will get used to it. You will learn important time management skills, test-taking skills and learn to study effectively. Then you will see your grades go up. After you learn your time management skills, you need to use your spare time to do research and/or other activities...
There are different ways of studying in different subjects, for instance, Bio is more of memorizing, so you can cram, or slack off a little (I study Bio a week before the exam). But for O-chem and physics, you need to be constantly doing problems; especially for Ochem, you need to understand the concepts to derive the mechanisms, don't just memorize. I am not so good at physics, I usually don't care how the professor derive the equations, I just need to know how to use them in the problem (you don't need physics in dental school anywayz..) So usually if the professor suck, I just ditch the classes, and use that time to do something useful.
I really don't do a lot of studying, because I think the important thing is not the quantity of your study, but the quality. Study smart, not just study hard.
Usually, to keep my grades up, every quarter I will take some other interesting classes (or easy classes), to keep my grades up, just in case if I mess up.
...these are just my method of studying, it may not be useful to everyone....
