Kids sharing rooms?

We have a 3 bedroom house- one master, two regular rooms.

We also have a 5 year old, a 1 year old, both girls. We have a boy on the way...

Not sure how we should make sharing arrangements once he’s old enough to move out from our bedroom..

Ideas?

Ps. 5 yr old is 50/50 custody with my significant other and their ex. 5 yr old is over every other week.