HELP! He's Pooping on my bed now!

I have 3 cats and my oldest male (Prescious) was peeing on my bed. I took him to the vet and he was unable to get a sample from him to rule out a UTI, but he gave us medicine to give him in case it is.
it's been two days with no peeing problems, but this afternoon i found poop in my bed! I don't know what to do! Should I lock them in the basement if we are not home? I'm at my wits end here and my husband wants to get rid of all three since we don't know who is doing the pooping now. Please help.

What's the basement like? Has it got soft places for them to relax, some light coming in, enough room to roam around, dangerous nooks or mechanics walled off? What might be different recently that was working previously? I would definitely have your cat rechecked for more serious problems apart from the UTI, but it sounds like one of them was making a statement on the bed - does it normally 'belong' to another cat? That would do it!

nothing down in the basement. there's only their boxes. it's been working fine for the last two years. the only thing that has changed is the fact that our niece stays with us on the weekends now.