Just read all 5 chapters available.
My general opinion on Yuni and Nanase is that they are emotional teens who are barely communicating and that are trying to protect themselves with their actions. I don't see them working long term, but I don't think one is worse than the other.
As for Taki. My take on her is that she wants attention, probably because her parents give it all to a sibling we haven't met yet. Her main goal seems to be to get Yuni for herself, but knowing this author, I'm not sure it won't be something more dramatic, like:
- Making Yuni and Nanase break up because she wants Nanase for herself. Essentially pulling a Namori
- Breaking up lesbian couples by cheating with one of them since she only gets pleasure out of Schadenfreude.
- Getting her dosis of self-esteem by having a Yuni ruin her relationship with Nanase and her being the cuase, which somehow would relate to the whole winter fungus thing.