Cat Peeing on Couch?

I rescued Bella 4 years ago when I found her at a mall parking lot all beaten up when she was 7 months old. For the first year, she did not pee on my couch, but has now been doing it for 3 years. I have to keep plastic shower curtains on my couches so she won't penetrate thru my couch. However, it's very nerve racking to constantly find pee and having to clean it up and keep buying shower curtains.

She does use her potty, however, every now and then she'll keep peeing on the same couch, same spot which I btw, clean with the pet products to use for cleaning pee. Her pee has not penetrated thru due to the plastic curtains I keep on the couches. Lately, she ended up peeing on the spare bed. OMG, what a mess and stink. I had to replace the undercover (which helps from stains or anything wet to seep thru, thank goodness the mattress was saved), sheets, and pillows.

She has two other male cats, one of which doesn't seem to like her very much and keeps on picking on her. I'm pretty sure this is a behaviour thing since it doesn't happen all the time, but lately has been. She has been fixed. I need to know how to stop her from doing this again. Please help.