If those three letters Nadeshiko got were from Ai, Miho, and Megumi (and why wouldn't they, to be honest) this means she has been in a pretty long relationship, since this takes place four years prior to the present.
Both events would be part of a hypothetical third season since Chiyo is the twelfth girlfriend, the first one not teased in the final minutes of the first season, and Momiji is the fifteenth.
All of this assuming each season introduces five or six girlfriends.
Because no one wants that.
We will find out how meta the adaptation is going to be the moment Chiyo nags everybody in the rooftop using a copy of the manga to correct Nano.
What I got from the flashback is that Yuuki wants to be mistreated by Marie but Marie never thought of that because she seems Yuuki as an equal to her.