Huh, I could've sworn I've read this before somewhere. Maybe on Danbooru? Regardless, Marisa, Patchouli and Alice are kind-of over the top horrible in this, so I really can't take this too seriously.
Actually, from what I understand, either Marisa was in an open relationship with all three of them and Alice and Patchouli took it too seriously, or Marisa was just leading the former two on. I'm leaning towards the former, since it seemed that Marisa was indeed sleeping with all of them and they all knew it, but Alice and Patchouli became insanely jealous (judging by their claim that "There's no meaning if I'm not number one!")
I should note that I haven't actually read the first one yet, which probably explains the situation better than what I've come up with.