I have DS & DD3 together. It was dd2 & dd3, but it wasn't working. DS wouldn't be alone in his room.

This is only now we have 4 brms. Before dd1 & dd2 shared, then the younger 2. Having the big girls together was a nightmare!

Both younger kids rooms have bunks. The little kids bunks are separated, so the dog can sleep with DS. Dd2's bunks don't separate, but she has room for a friend over or what ever. Dd1 has a double bed. If we have family stay, they take her bed & she gets the lounge or dd2's spare bed.

As they get older, they change their minds, & as their personalities develop you'll know what is the easiest for you.

Eta; I'm pretty sure government housing here in Aus has similar rules. Just not law. I looked into it when they told me I wasn't over crowded, so could be moved down to a 3 bedder. I thought it stupid that DS had to have his own room, but that dd1, being pretty much a teen, didn't matter & had no rights to privacy.