We have a three bed. Eldest son ( nearly 3) in the biggest room newly decorated for him. Baby daughter in slightly smaller room ( but not significantly so). If we had a third baby not sure what we would do about the bedroom situation. We have a study downstairs. This could potentially be a bedroom although feels odd. So if we had a boy would you put him in with the eldest son? Or with younger daughter? At what age is it acceptable to put them in together?? Or move the eldest downstairs?. If we had a girl I assume we would put the girls together but again not sure when it is safe to. Can't leave my baby girl alone with son for a minute at the mo!!!!.
I tend to think seperate bedrooms for all is very a Mumsnet thing. In real life, lots and lots and lots of people see children sharing as the norm. We live in a two bed. We have two children - boy and girl. I'm pregnant with dc3. At some point we will always have needed to move because DD and DS will need privacy when they're older. Whatever DC3 is will share with older brother or sister accordingly. Until then they can all share together. As it is right now DS 3yo shares with DD 17m. Baby will stay in with is probably for a year (we co slept with all of them) and we have bunk beds with a small double underneath. They're split at the moment but I've no problem with any of the children co sleeping together - provided they actually sleep, that is Their room is really large though, big enough for three singles.
Think the key is to be relaxed about it and ok with sharing. It's entirely normal bunk beds are brilliant and in the early years I think it's more about who sleeps better with who.
Sharing rooms is totally normal, and we will have the same situation if we have a third. My thought is that you put the two of the same gender together in the larger room, as soon as it's practical to do so. So whatever you have you could move DD in with DS for the time being, then when new baby is ready to share you move the one into the little room, and redecorate as appropriate. Starting off with gender neutral paint on the walls would be an advantage, but not a huge issue!
I would just put the third baby in with the sibling of the same sex. Your study downstairs would be better used as a playroom/toy room than a bedroom imo. Kids "needing" their own bedrooms is a very new thing.