My gpa is currently at a 3.68, but if i try to graduate by next year(and take 18 credits per semester) theres a chance that it might slip to a 3.45.
Don't let that happen. 18 hours is really not that much, unless you are taking some seriously killer classes with balls-to-the-wall professors. In which case, don't do that.
Now I'm thinking about spending an extra semester at my university and taking 12 credits each semester(for 3 semesters) instead of 18 credits each semester(for 2 semesters).
By doing this I would be able to work on my extra curriculars and also be able to bring my gpa up to a 3.78.
You mean you
could raise your GPA to a 3.78. There are no guarantees. If you shorten your school year, you may be able to take a trip to europe or something like that with your time off. That would be an excellent idea and would look good on your app (if you did something productive). On the other hand, if you plan on just not doing anything post-graduation, I'd say don't do it.
I just wanted to know whether med schools would frown upon me taking a low amount of credit hours during my last few semesters.
Yes, they do. But this is a rather minor concern. 12 hours/semester is low. Take fun classes and bump that number up to 15+ hours and your problem disappears. You are at college to learn things, not just to get into medical school. When are you planning on applying? Remember, once you get accepted, you have a year before matriculation. Cherish that year. Hold it close to your bosom and love it with all your heart.
Does graduating in 3 years really make a difference?
Yes. But that doesn't mean it is a good idea. In general, med schools prefer well-rounded, educated individuals. Taking an extra year and actually learning a few things outside your major may benefit your admissions, and will definitely benefit you as a person.
It is an education, not just an application.