How do I stop my cat from peeing on the couch?

I just adopted three kittens. They are not spayed/neutered yet because they are only nine weeks old and was told I had to wait another few months. Recently they started peeing on the couch. Not always, only sometimes. I have tried putting down foil, getting additional litter boxes, changing the litter and I cleaned the couch with every imaginable odor eliminating product that I can find to no avail. I have two older cats and they have not starting peeing over the kittens spot...thank god. I can't take any more please help.