They may take a little longer to find their place within the pecking order. It sometimes takes as long as a month before new and old hens are fully settled with each other. Once the new hens come into lay the dynamic will almost certainly change. You could try taking the older two out for a few days and then re-introduce them, so they are the newbies. Another possibility is to spray all the hens with a weak vinegar and water solution so they all smell alike.