Unlike a filler like Juvederm or Restylane, Sculptra is best used for more volumetric filling to replace the volume lost by the aging process. By filling hollowed cheeks, the nasolabial folds will actually tend to improve in a more natural way. I have no objection to putting some filler into the nasolabial folds, but it should be done modestly, or else a "monkey-snout" look will result. Even many young people have distinct nasolabial folds - so trying to flatten them by pumping them up with more and more volume does not make someone look youthful, it just makes them look weird. It seems your doctor has a good plan to put more Sculptra into your cheeks during your next session,.
I wish you had posted a photo but it sounds from the description that you need more Sculptra in the cheeks. Injecting Sculptra in the cheeks gives a nice lift to the face and lessens the nasolabial folds. As with all cosmetic procedures please find a board certified dermatologist with a great deal of experience with injections and cosmetic procedures.
If there is still an area of hollowness present in your cheeks, then it would definitely make sense to place more Sculptra there on your third treatment and leave the nasolabial folds alone. Generally speaking, when the cheeks are improved and filled, this normally will make the nasolabial folds look better. From the description of your Sculptra treatments, the plan for your next treatment sounds correct.