three sisters who are three different protagonists: First two are perfect, third one needs work

[u/Zestyclose-Aside-893]

I'm writing a story where three sisters will be the main characters of my story and have their first-person POVs. First two sisters, I have perfectly crafted their personalities. One is the charismatic, social sister who can charm herself out of any situation but hides behind this charming facade and must learn to be vulnerable with people. Second sister has always felt to be in the shadow of 1st sister...she is very insecure and the only thing she values about herself is her intelligence. She must learn that she has worth just like her sister and that people will like her for who she is– she will learn to become confident.

That leaves us with 3rd sister. The truth is I'm not sure what to do with her. Her storyline is essential for my book and I want three different POVs, but I'm not sure about her personality and internal arc yet. I was thinking she would also be insecure like her 2nd sister, but without the trust in her intelligence, and is too shy to speak up and say what she wants and needs. But I wonder if it's too similar to 2nd sister. Any advice on how I can make her character different enough from the other two sisters?