I just look in Patreon and there is a pool saying that the game will not have a "harem" ending, but it can have a three-person relationship ending? Well, you know your character better than me, and maybe in the next update there will be something that makes it make sense, but I really don't see how it couldn't be an open relationship, like, they don't need to all be with each other and living together, the Mc could have a singular relationship with each of them, so far the only one that I see having a problem with it is the older blond sister, but she is part of one of the troubles, so I don't know.
Just to clarify what I mean, there could be the two separate three-person relationships and the relationship with the youngest housemate, they don't need to all interact with each other, each relationship would be separate, but as I said, you know your character better than me, and maybe there is an explanation in the future update.