My cat won't pee in his litter box!

My cat, Walter, has decided to selectively pee in his litter box. He's taken to peeing in random places around the house, but strangely, will still pee in his litter box too!  We can't figure out why. We have two female cats as well, and we've been keeping them away from Walter's litter, just to make sure that he doesn't feel territorial. It's not working! 

Help! 

How many boxes do you have? With 3 cats,there should be 4.

Take him to the vet. He may have pain urinating & obviously associates the box with pain. Hence the peeing elsewhere.