How does this person know that the others are happy? That is an assumption.
What is her purpose of having a relationship with more than one person? Why even have a relationship at all? Just be with the person who you want for the day.
There are many people in this arrangement that will be hurt in the future. A relationship means responsibility towards the person in the relationship. Now if this is an actual relationship, I would call this communal living. Well that is not my cup of tea. She can do what she wants.
Now society will scorn her. Is that fair? It is the way society is.
Personally I would let her do her own thing but she would forever be in the "I would never want to be with her catagory." She has to rationalize to make this ok.
So my question is why not just do what she wants and not have relationship. Enjoy what she is doing until she is ready. A relationship is by definition commitment. No one said she needs to be in relationship. 3 relationships is farse. This is just multiple casual what ever you want to call it. That is fine and a lot of people do that. Just call it friends with benefits.......Whatever, but it is not a relationship.