I think your misunderstanding just comes from the use of the word "room". To make things simpler to understand: room does not always mean bedroom. There are many rooms in a house: living room, bath room, bed room. Roommate simply means someone you share some of those rooms with (usually bathroom, living room, kitchen...) but obviously not the bedroom.

So, in the case of "6 of us in a 3 bedroom apartment", then yes, they shared rooms, but it's not always the case (I would've expected 3 people in a 3 bedroom apartment). Perhaps parent meant 3 couples (6 people)?