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

Usually in these cases, parents would let the oldest daughter have her own room and the 2 younger ones share another room. The logic is when the older one moves out in the future, the next older sister can take over. Alternatively, you can find a way to split one of the bigger rooms to become two smaller rooms by using partitions -- that way there will some semblance of privacy. That's what my dad did with my brothers' room when they grew into their teens.