You really need to purchase a black light and an enzyme cleaner. Sounds like you're going to need an industrial size bottle. We use Nok-Out, and it's available here: http://www.nokout.com/pet_odors.html
If you haven't been cleaning the clothes, the mattress, the furniture and the carpets with an enzyme cleaner, they are going to continue to smell the pee and continue going there.
Go through your house with the black light - any orange splotches are kitty pee. DOUSE the area with the enzyme cleaner - and I mean DOUSE. Down to the floor boards. If you did it right, it should take a couple of days or more to dry completely. If they've gone repeatedly, you may have to douse the area again - which I would consider doing anyway. Wait until it dries, then do it again. If you can, cover wherever it is with aluminum foil while it dries - this will discourage continued use of the same place - even it if means your home is half covered in aluminum foil. If it's on couches, keep some throw blankets around to toss on there when you want to sit on them. Pick up the blankets and put them on a shelf or someplace kitties can't access when you're done sitting.
Take anything they've peed on - towels, blankets, throw rugs, etc. and add the enzyme cleaner to the wash.
For your mattress, you must soak the areas where they've peed. Take a couple of towels and lay them over those spots then make your bed so the towels can absorb the wet while you use the bed.
Make sure you have a minimum of three litter boxes. I'd go for four.
Consider purchasing a product called Cat Attract. I believe there is now "cat attract" litter - we purchased an additive that you use to cover the litter. It helped our female feral that didn't have a medical problem.
I'd also consider purchasing Feliway spray or plug-ins. This will help destress the kitties. You can also consider Flower Essences - this helps some cats, it doesn't help others. But it seems like a highly stressed situation, so it may help. Feliway and Flower Essences can be purchased here: http://www.catfaeries.com
Also, to discourage continued use of the bed, cats HATE the smell of Oil of Olbas. Purchase it from a health food store, put some drops on a damp tea bag, make a couple of those (I'd do like four), and place them around on the bed - until you want to get in it. It should be enough to prevent kitties from wanting to be on the bed, let alone pee on it.
I'd also make sure to give each kitty some special alone play time. At least 10 or 15 minutes a day. This will help diffuse stress.
As a very last resort, I'd consider separating them and retraining them. Try separate rooms. If they use the boxes when they're alone for a few days (good time to let the enzyme cleaner all over the house dry), try letting them out for several hours at a time. Increase the amount of time they're out of the separation rooms each day. Any time you see one of them using a litter box, praise them to high heaven and give them treats.
Hope this helps,