There are several medical reasons that a cat may pee outside the box. If it is a URI or crystal/stone formation, the reason they use the floor or other places is that when they pee in the box they feel the pain and associate it with the box (they do not realize that it is their bodies hurting, they just know the box = pain). But a cat with diabetes or Kidney problems may also miss the box for a toatally different reason. They may attempt to use the litter box, and sometimes they may succeed, but other times they may have to go so badly that they can't make it.

Your cats need to be checked for medical problems. I know that with multiples, narrowing down the culprit may be difficult but it will be necessary to solving your problems.