We have multiple cats and it has been a problem for us as well. It is a catch-22...you want to save as many as you can, but there are many problems the more cats you introduce. Sorry you have had to go through this as well.
After getting ours checked out, we decided that we are just going to wait it out and completely regut and remodel when they are gone. It will be at least 5 to 7 years yet, and in the meantime, we will just keep cleaning it up as we go along. We have tried Feliway spray, Feliway room diffusers, Valium, Wellbutrin (both making the aggressors woozy), Nature's Miracle, baking soda...you name it, we've tried it. Our hardwood floors are ruined in spots, but it will all work for the good as the biggest problem area is in the kitchen and we were planning to put in all new flooring/subflooring anyway.
What you can't do is punish the cat in the act of urinating. That will really mess with their minds, as this is a normal bodily function...and they can't distinguish from being punished BECAUSE they are urinating as opposed to WHERE they are urinating.
Try different kinds of litter as well and keep the boxes as clean as you possibly can. Rule I've read for litter boxes is one for every cat plus an extra box, so you would need 3. They also might smell another cat outside, as this is the case in our neighborhood. Ours never go out but lay in windowsills and look out the screen doors, so they get plenty of scent exposure. This can set off spraying behavior as well.
It is a sad problem--just know you are not alone, and we are not BAD people.