They are all having vet visits within the next few weeks, for their yearly checkup. I will talk to the vet.

I also looked on a cat forum (perhaps more accurately a cat-owner's forum?) and found some advice about using something called Feliway. I sprayed it around the litterbox, but once again this morning she used the floor.

The advice to other people with similar problems, on that forum, was what you suggested: isolate them from each other.

Also we're supposed to have 1 box per cat + 1. With the new box I'm up to 2, so I'll go ahead and get 1 more, which will still be 1 short, but I'll see how that goes. I'll put the 3rd one upstairs, spray with Feliway, and keep Old Cranky up there. She won't mind being confined to one room because she doesn't get around much. Young & Restless can have the rest of the house, and Old Spacey doesn't seem to care who is around her. (That's not their real names.)

Three cats is really too many, and I knew that at the time, but what can you do? But if I see another stray I will steel my heart and turn my back!