The rule of thumb per the "experts" is one box per cat plus one more. It is also recommended to have the boxes available on each floor of your home. That being said, I only have 5 boxes for 6 cats and have never had a problem. (knock on wood) I also use Tidy Cats Scoop and provide one box that is uncovered. The rest are covered and the uncovered is the least popular for my cats. What Riley was referring to as far as health issues was the possibility of urinary tract infection or urinary crystals. Inappropriate elimination is often the first sign of a health issue. Even though all 3 are doing it, there may be one with an infection and the other 2 just smell it there and decided to join in. You are lucky they chose the tub. As much as I am sure you would prefer they did not, that is better than the carpet. I would keep an eye on which one seems to be peeing more frequently and may also seem to be straining to pee. If so, that one needs to see the vet unless you can get all 3 in to the vet for a check up. Second most common reason is litterbox protest. So even though you think they are clean, the cats nose is far more sensitive and they may think otherwise. So try one more litterbox in a convenient location.
"One cat just leads to another" - Ernest Hemingway
Actually, as someone with 10 kitties to watch over, inappropriate urination and urinary incontinence are two different yet similar conditions; one being behavioral in nature and origin, and the other physical. Therefore, you don't want to scold if the cat or cats all have a condition that makes them powerless to control urine release anywhere but a most comfortable place for them.
From what I have read, if you have tried ensuring the cat boxes are always clean, if you have enough available, and any other suggestions made by others have failed, try putting the litter box in the bathtub and see if they will use it there.
Inconvenient though that may temporarily be, once they get the "hang" of using the cat box, you can inch it closer and closer to where you really want it finally positioned. At the same time, frequently spray down your bathtub with a urine odor eliminator to discourage reuse of a spot that once had a familiar smell.
If that still doesn't help, seek out the advice of your vet to rule out physiological problems, and/or get help with behavioral problems, as needed.