I would advise against retaking courses you've taken already (unless you failed). It would be very unfortunate to retake a class you got a C in and only end up with a B-...that's not impressive. Instead, I'd suggest taking higher level bio/chem/exercise science courses that aren't prereqs but will still factor into your science GPA/upper division GPA. Maybe look at your schools suggested courses (but not mandatory prereqs) and see where you think you could do well (e.g. organic chem, biochem, higher level anatomy/phys, exercise phys, etc.)
And if you're taking the same class three times, then that would definitely raise some red flags. Retaking a class once is one thing, but taking it a third time is a bit excessive.