We recently added a new cat to our three-cat (now four) household. Our oldest cat is wonderful and gets along with everyone, cat and human alike. The problem is between our newest, youngest cat (the "victim") and our two other cats (the "bullies"). The first few weeks were fine; some hissing and occasional fighting, but that's to be expected. It's been a few months now, and things have only gotten worse.
Over the past couple of months, the bullies would stare her down, chase her, back her into corners, and tackle her, at every opportunity. The victim always retreats to our bedroom, where she usually hides under the bed. She's gotten attacked while eating, while drinking, while playing (with us!), and while using the litter box. Now, she never leaves the bedroom at all, and will hide under the bed unless we are in the room and the bullies are not. We have had to start feeding her behind closed doors, add a water fountain in the bedroom so she can drink without getting into a fight, guard her from the bullies while she uses the litter box, add a third litter box, and physically handle the bullies to remove them from the area after a confrontation. One of the bullies even hissed at me after I had to stop him from interrupting the victim's bathroom break.
I believe that one of the reasons the situation may have gotten worse is that the bullies can no longer be in the bedroom very much anymore. Before we adopted the new cat, they would spend time in there, sleeping during the day and hanging out with us at night. But due to the constant bullying and attacks, and with the bedroom being the victim's only "safe space," they have to be removed from the bedroom all the time now. One of the bullies has even started invading her space under the bed; when that happens, I can't even get underneath to intervene.
I'm running out of ideas for how we can deal with this situation. We have tried administering an herbal supplement to the bullies in their food, but the behavior has only gotten worse. Aside from an occasional water bottle spritz and picking up the bullies to remove them from a room, we do not punish them (I've been told that cats don't understand punishment and it makes them even more upset). We both work during the day, so there's not always going to be someone around to rescue her from being backed into a corner or block the bullies from interfering with a pee break. I'm worried that we are soon going to start seeing actual injuries or finding poops under the bed because she's too scared to come out.