I generally agree that 'harem', (or more accurately, sharing, as harem has a lot of meanings) isn't suitable for Hillside.
However, I can think of one exception that is plausible, a Lucy and Charlotte share MC scenario.
I imagine it would go something along the lines of being in a relationship or involved with Lucy. Lucy realises that MC is the only man Charlotte trusts or feels affection for. Lucy, overcoming her previous past jealous towards Charlotte, encourages MC to pursue Charlotte romantically so Charlotte has a chance at happiness too. Shenanigans ensue and MC ends up with in a relationship with both.
I'll emphasise that this is plausible with the story as currently written, not necessarily the best or most likely outcome narratively.