Would products made and packed at a central location, then sold direct from three market stall sites, qualify as PPDS? Or should these be prepacked products?

Yes, it would qualify as PPDS if the business that is packing the products is the same one that is selling it from those three different market stalls. But if the packaging of the food was done by a different business and then was sold from the three market stalls, this would be prepacked food.