LOL! "obsessive ex-boyfriend" what a great description.

I think if she's under the bed, and he's sleeping next to it ... they are on their way to working it out already. Give it time.

Thanks, pamghatten! I feel badly because guest cat has been staying with me for years, for up to a month or two at a time, and I don't want her to be so worried now that she decides it's easier to just hide all of the time. At the same time, I don't want our cats to have to all be locked out of a room, nor do I want guest-cat to have to be locked in unnecessarily.

I just wish that the lummox would understand that she doesn't want to be friends with him. He was still an older kitten when she first came to visit; maybe he thinks she's his mother.

I think it depends on how 'old' she is. Even though the guest cat has been at your house before, she's now coming to a place where there are other cats. It can be difficult for a cat to adjust to changes and even more so when they visit another home and enter another cat's territory.
Even if she were to adjust quickly this time, she would have to make this adjustment every time she visits.

I would worry that she's not using the litter box regularly nor eating normally, which could lead to health issues. If she prefers a certain room I'd let her have it to herself, especially since you said she's a loner.

I would get a harness and a leash for the maine coon mix...problem solved

I agree with annz. My cat is the same with our dogs, she's old now and they are just too noisy and rambunctious for her to be around so she spends a lot of time outside now sleeping on the sunlounge. If she comes in and finds a quiet place to settle down I just close the door to keep the dogs out of the room so she can have a snooze in peace without the dogs pestering her. She's fine with it and I suspect your visiting cat will be quite happy to be just chill out on her own for a lot of the time.

I'm guessing she spends time alone at her own place and isnt used to living with other cats so she'd be used to having her own time to herself anyway. She's only going to be there for a few weeks so I dont think getting her used to your cats is really that useful and she's stressed out being around your cat. Cats are very independent animals so they dont need to be around people all the time and they do enjoy their own company so I would just keep them separated as much as possible.

one other thing is that when cats are stressed (and being away from home is stressful) they prefer small spaces because it makes them feel safe (such as being under the bed because she's stressed around your cat), so she really doesnt need or perhaps even want a run of the house when she knows there are other cats are around.

Thanks everyone for your replies! My husband said she was out and about for a bit last night; I told him that if she is not at least out from under the bed more often in another day or two she's getting her own room back. (My husband is the eternal optimist, I am a pessimist.)

I wish I knew what she wanted; she doesn't seem to mind the two females and if she's out when they come in they pretty much ignore each other. And she does like to wander the house - until the male comes galumphing around.

He does have a harness; I hadn't thought of that. Before we got the two newest, we'd switch cats at night (guest got the room during the day while he ran the house, in evening and at night she got the run out of the house and he went into the finished basement). Since the two females don't seem to bother her maybe that's an option again too.

Thanks again, everyone. Ideally, of course, they'd co-exist nicely; I just worry that he's never going to lose interest, she's going to be intimidated, and we should have just kept them separated.

Oh, and she is eating, drinking and coming out at some time to use the litter box.

I probably worry too much but really, this is the only place guest cat can stay while relative travels - nobody wants her to be in a kennel for two or three weeks at a time.

I just wanted to post a quick update in case anyone reading might have a similar issue. Saturday I was tidying the bedroom and had had enough of male cat underfoot trying to find a good vantage point to stare at guest cat, so I put him in the basement. No sooner had I done so than guest cat came downstairs and wandered around the house - so it was obvious that her problem was with him and not the other two.

Yesterday, whenever he'd "stalk" guest cat I'd reprimand him and put him in the basement for an hour or two - by last night he was very studiously ignoring guest cat (he'd come upstairs while she was there, and apparently deliberately lay down on the opposite side of the room and look away from her). Guest cat has been out lying in the sun this weekend and is currently roaming around so I am cautiously optimistic. Thanks for the advice!

Congratulations!! Seems like a very good solution and boy cat is a quick learner!! Good job!!