3 cats, 1 with a problem

Hi, I am new to this site, and my first post, so I hope it works. I have 3 cats, one which is adopted from the local shelter. The newest cat is wonderful, purrs like a Harley Davidson, hence her name Harley, but is terrorizing our second cat, Tar. Tar has been hiding from her, not getting to eat, hiding in the closet under the shoe shelf or behind the couch and generally not being her normal self. This has gone on for about 5 months now. We are about at our wits end, since Tar has just started peeing in the kitchen within the last week. We have 3 litter boxes (cleaned daily)and don't know what to do anymore. Our first cat, Spook and Harley get along great! Tar and Spook still get along fine, when Tar comes out of hiding. I've taken Tar to the vet's and she is fine physically. Mentally I'm starting to wonder and it makes me really worried about her. Any ideas of what to do next?

It sounds like Tar is turning into a poriah kitty. Harley knows she is scared, and will continue to try and put her in her place. Harley needs to be the Alpha female, and Tar doesn't know how to deal with it. She is probably peeing outside the box in fear that Harley will get her in the litter box. I would pick a room that Tar can have to herself. I would let her out to play a few hours at a time, and as soon as Harley becomes aggressive with her, put her in her room for some peace and quiet. In the meantime, you can try to work with harley by taking her into another room when she becomes aggressive.
This may or may not work, but you can try putting some Dr. Bachs rescue remedy in thier drinking water every day. It doesnt' work on all cats, but it helps with stress and is all natural. It may be benificial to Harley and Tar.