Your boyfriend has a very good girlfriend, most likely they have known each other since childhood, but you are very upset when you see them talking or laughing together or when he says things to them that they would never share with you.
Stop worrying and learn to understand their relationship. If you do not have concrete evidence to the contrary, the relationship between them is simply friendship. Strong, indeed, but only of friendship.
He tries to keep a good connection with her, he doesn't look for a knot in everything he does and he doesn't look for a reason to quarrel between the two of them. It may seem unnatural, but since they've probably known each other for longer than you know your boyfriend, if you insist on breaking up with them for no reason, you might have a surprise - choose her instead.
The best thing you can do is become friends with her, get to know her. The example follows: "Keep your friends close, and your enemies even closer." If, as you learn more about her, you get to know her way of thinking and personality and earn her respect, you will know how things are in your trio.
In addition, if you are friends with her, you can find out more details about your boyfriend, who he was careful not to tell you and may come to your aid when you quarrel with him. Then, if it bothers you so much, you can sometimes talk openly with your friend about it, without speaking ill of her, but trying to highlight her weaknesses in a subtle way. You don't want him to perceive you as a jealous woman for any reason. Men don't like very possessive women.
Be careful, though! Don't make his girlfriend a subject every day! Take his time and try to treat him as nicely as possible, make him feel loved! In this way, you will be able to make him consider you the ideal woman, understanding and with his head on his shoulders! In time, you'll get rid of the "problem"