Well yes I can only state what is by design this is suppose to do but I hear you how you want it to work.
Multiples do not show on the features but on the following scripts showmembers and the thumbnail strips in showphoto they are treated as separate photos. Just how Michael wrote this feature. 3 inserts in the database and use of the storecat function to save diskspace by using the ability to only store one photo on disk to represent up to all 3 photos in separate cats.
Perhaps this is about as clear as I can be to say how to modify your code.
Sorry I can not post php files it is against company policy all I can to is post code blocks
In showphoto.php as stated lines 185-237 you can replace with this block
Content visible to verified customers only.