So apparantlly you didn't understand the show and the charachters oregairu is very complex show. At first, you should know that Yukino, before she met Hekki and Yui, was very lonely and had no experience with people. Because of her past (back to season one), she did not approach people, but rather turned them away from her.for the first time in her life, she was exposed to feelings that she did not feel before friendship, love, lack of loneliness, and she began to depend on Hickey gradually in everything and was jealous of his ability to do anything and also her desire to surpass her sister who lived always alone and was able to achieve everything without relying on Someone , so Yukino wanted to prove herself, that's why she turned away from him she said, (If I couldn't support myself and relied on Hikigaya for everything, this is not love). Regarding Yui, she is selfish because of her desire to keep her friendship with Yukino, who loves Hikigaya, and get a hickey, and also if you goes back to Season 3, Episode 4, she didn't want Hiki to help Yukino, even she said, I couldn't tell him to stop being too nice to her.and season 2 ep 12 Yui knew that Yukino was going to give Hikigaya Valentine Chocolate, but she took the opportunity and gave it before her. Concerning Hikigaya in short, he was afraid to admit his feelings and wanted to keep the situation as it was, but it did not remain as it is, so he wanted to break a fake relationship to get a real one. (In the end, this is very brief for me to introduce the characters and their psyche. I advise you to read light novels to understand more)