1) If you have 3 cats, you should definitely have more than one litter box. Do you want to use a toilet that is already full of someone else's poop? Neither does a cat.
2) Your cat is not an idiot. It's probably doing it on purpose because it's upset about something. Has anything in its routine changed recently? I started buying different food for my cat a few years ago, and she started peeing on everything. I started buying her old food again, and she immediately stopped peeing on stuff.
3) If your cat isn't doing on purpose, it might be legitimately sick.
Also, cats like to pee in places they've peed before, and even if you wash your clothes/sheets/whatever, your cat can still smell it, even if you can't. There are pheromones in cat pee that will not come out with traditional detergents.