I have to eventuall take these classes for my major, but I would rather get the simpler ones done during the summer. The lectuers & labs will be comboned diring the summer semester, so Ine class alone will be 3 hours or more. I have to take microbiology, physiology, medical chemistry, and human autonomy with laboratories.

None if these are easy classes which is why it is a great idea to take them in the summer. Take one at a time so you can focus on it completely and not also have other courses to study.

Which is hardest depends on the student. For me, the easiest to hardest rank would be physiology, then anatomy (not autonomy), biology and finally chemistry. I am not great in advanced math though , and you may feel differently . I would not take more than one lab course at a time. They all involve a fair amount of memorizing terms, scientists , concepts, parts of cells or body parts and systems , etc., and it would be quite difficult to do that in two advanced subjects in a short summer term.

If each of these courses has to be taken and none is a prerequisite for a class you are taking in the fall, then I'd look ahead to the class schedule. If one if these classes is offered at a time in the fall that is inconvenient, then take it in the summer.