I just came off of my third semester with 3 science classes (each with a lab), 1 higher level math class, and 1 history class, 20 credits. I was kind of cornered into taking that load, but I was able to end that semester with a 3.9.
Would I have done that if I had the choice? Probably not. Because when exam time came around, I'd have to be jumping from studying from one exam to another, and most of the time, I'd have 2-3 exams all in one week, and then more exams the next week. But what I did take out of this was time management and being efficient. In a sense, I'm kind of glad that I took all that because it really helped me out in the long run, as well as boosted my GPA.
But again, if you're studying for MCAT, like what others said, I'd highly suggest you stick with a lighter load. Med Schools aren't going to know about your course load, unless you tell them during your interview, or some how squeeze it into your PS, which I wouldn't rely on too much.
Best of luck