First I want to say I admire your tolerance and patience. I am so thankful that no where in your post was any comment about "getting rid" of your babies! GOD BLESS YOU! I work in a shelter so I see many cats come in the door for far less reasons than this.

I'm not sure where you are located but there is a new litter that Pet Supermarket sells that is money back guaranteed to draw cats to it and stop outside the box peeing. I don't put a whole lot of stock in it but they do give you back your money if it doesn't work.

You have three floors in your house? Where are your litter boxes? Are they easily accessible on all floors? Have your girls been checked for URI's? I know its highly unlikely that more than one would have it but hey you never know!

I have one female named Maggie Sue that will not use a litter box for anything! She is far too good to use a box that is shared by others! She uses the floor right outside the box. Thankfully I don't have to deal with her peeing on counters or anywhere else. She does on occasion use the human toilet! Yes, really she does. She will straddle the seat and pee very proudly. Now if I could just get her to do that all the time!!!

The only real solution that I know to work is retraining. Generally that means confining them to a small area and slowly expanding it. I know you don't want to do that and I don't blame you but other than that I have no idea.

Are you sure its the girls and not territorial marking from the boys?