Is the cat fixed
has this been going on for a while
have you tried different cat litter
has the cat been to the vet for a check up to make sure it is not a UT
infection

For three cats you should have four litter boxes.  Any time a cat
urinates outside of the litter box, a vet trip is in order to check for
an infection or crystals.

                 ---MIKE---

I'd add two more litter boxes.    With three cats you should have four
boxes.  

The first thing to do is take the cat to the vet to rule out
any underlying medical problem.

First, take the cat to the vet to make sure it's not doing this for medical
reasons. It is rare for only 1 cat out of 3 to be doing something like this,
so my first suggestion is to take it to the vet. If it's a bladder
infection, once the infection clears up, the cat will likely use the litter
box without you having to do anything. Plus, you should have at least 1
litter box per cat in your home. All boxes should be in low traffic areas
for the cat's privacy. Cats prefer to have a nice quiet place to do their
business.

I also have 3 cats and have 3 litter boxes. My older cat has become more
finicky about wanting the litter boxes as clean as possible so I throw the
litter out twice a month and thoroughly clean the litter boxes with very hot
water and antibacterial soap. I put brand new litter in the boxes and
everyone's happy. I scoop the boxes every day and every other day I add a
little baking soda. I'm not perfect, when I'm sick I don't get to their
boxes every single day, but all the other work I do helps to keep their
boxes very clean and hygienic so they always go in their boxes and not on
the floor.

If you discover your cat doesn't have anything medically wrong with it and
pristinely clean boxes don't solve the problem, I suggest retraining your
cat. There are many websites out there that can teach you how to litter
train your cat. BUT, you need to make sure your cat isn't ill. A bladder or
urinary tract infection is painful for the cat and makes them very ill.
Acting out by not using the box is one of the first signs a cat is ill. Be
safe and take him/her to the vet. Good luck!

I had the same problem, i have 4 cats and they pee in the tray, or in the
bath tub ... then i got a 5th Tray and now its solved its-self. Weird, but
it works... and yes that kitty litter costs a bomb!

Good luck