Quite a strange spot to place a dramatic flashback as we barely know anything about Rin, but whatever.
I was really confused by how long was the flashback timewise but it seems there was about a year (a couple months less maybe) between the friend's failure and the girls meeting under the tree and wow, one year is quite a long time yet neither of them could sort out their feelings and just let things go.
I have a bit of a problem with the fact that it's not just that Rin can love more than one person at the same time, but that she can't seem to be able to love less than two people at the same time, and she apparently treated her love for each of the girls as love for both of them at the same time? I'm not poly so I'm not sure but I guess this is not how it works.
I'm curious about this because it seems like I'll end up seeing the story more as an contrived comedy than a serious romance or drama.