My brain is at a stand still from thinking through this. We currently have everything stripped out of the kids' rooms today for carpet cleaning, so I am wanting to put things back where I want them when baby 4 comes in 3 months.
One of my kids is downstairs so she is out of the equation.
Right now we have a master bedroom, and 2 br upstairs. We have a 4 yr old and 19 month old. Both of them have some health issues that make night time a little tough, as they wake often needing some assistance.
Currently we have a twin mattress set on the floor of our 4 yr old's room. She starts the night there, and if she feels okay, stays there all night or otherwise wanders in with us between 1-6am.
Our 19 month old's room has a full sized futon mattress on her floor. She starts the night in there and wakes usually before we are asleep 10pm-12am or so and comes in with us for the night. She is a challenge, sleep wise, and is due at the end of the month for a sleep study. When both little ones are in our bed it is awful and no one sleeps. Sometimes I go with the 19 months old to sleep on the futon mattress. Sleep is not great for me there.
When our youngest was a baby we had an arms reach cosleeper. We now have a sidecarred crib that she won't sleep in any better than on the futon mattress in the other room. It holds laundry at this point.
I often wonder if our answer is to work on getting the 4 yr old to stay in her bed. Or the 19 month old the sleep more in the sidecarred crib. But in 3 months we will have a 4th and final.
New baby usually has the cosleeper option, although they are right next to me most of the time. I also usually set up a crib or something in the baby's room (although this one does not have a solo room) to have a quiet place to put baby down for naps when I want to shower, do something with other kids that I can't wear baby for.
We have never had a child that used a full sized crib for much time. We have a crib, minicrib, packnplay, arms reach cosleeper, extra twin mattress set, the futon mattress set, toddler bed. (a lot of sleep stuff considering that our kids always sleep with us, lol).
We struggle with all of this because our youngest two's health has involved a lot of night time parenting so we tend to move bed situations a lot.
Here are my thoughts:
1) If my 19 month old continues with her night waking stuff, am I better off making the crib next to our her primary space and encourage her to stay there? Or do I just throw her between us and side car the cosleeper for new baby?
2) Do I put up the crib, minicrib, packnplay in the 19 month old's room for a second sleep space for the baby? Or will she just wake the other and vice versa during naps and we should keep them apart?
For those of you will more than a comfy number of cosleepers (and limited master br space) how do you structure that?
My kids are 9, 5, 2, and 5 months, all sleep with a parent either nightly or from time to time. 2 of mine have special needs at night, one with asthma that has troubles at night and another with an anxiety disorder and insomnia. 2y just flat out isn't interested in sleeping alone, he doesn't have his own space to anyway. We have a 3 bedroom house and have two family bedrooms and one room with bunk beds. My Dh takes the downstairs room which is across the hall from the girl's room. He gets the 2y at night and once in a great while the 5y. I'm upstairs in the real master. I have the baby every night. The girls usually start off the night in their room, this only works if both of them are home (DD1 travels often) and DD1 isn't freaked out about something. When the 5y wakes up between 10-12am, she comes upstairs in crawls in my bed. I also have a twin mattress in there that is free for DD1 who does use it about every other week and then DD2 will sleep on it when DH travels and I have DS1 in my bed. 
And that is our motto these days, we will try any sleeping arrangement out there that gives the maximum amount of people, the most sleep possible. Separating the kids really helps my family to achieve that. Each take a kid now for a trial period before new baby arrives and see what happens. Worse possible case, it can't be any worse then how it is now? Right? When we want adult time, we start the girls off in their room, put DS1 in DH's bedroom, put the baby on the couch and sneak up to the master or claim the living room as ours (umm, if you do this then it really helps to turn off all lights so if a night walking child wanders out... you get the idea...) and put the baby to bed in my bed. 
