If you have multiple kids and stay in a simple 4room HDB, how would you share their rooms? I happen to have 3 girls.

It's better to let 3 of them in a same room, to be fair. Easy not for one to do things without letting parents or sister know too. The other room can used as study room, so won't disturb other sleeping too, there is time some will need burn midnight oil while some don't do.