I’m in a triad relationship. Whenever my girlfriend feels irritated, sad, or any negative way, she always goes to our boyfriend to find her sense of calmness. I know it sounds bad...but I wish I were able to help her like he does for her. I end up feeling like a useless girlfriend because when she’s like that, I say something to try and calm her down and she immediately just asks if our boyfriend is free to text her so that he can do his calming down magic.
Have you talked to her about this? During a moment when things are calmer, bring it up gently. You can say something like “I’ve noticed that when you want to be calmed down or cheered up, you always go to Samueth. Is there something I can do to make you feel more comfortable coming to me? When you’re not feeling good, what are things that help you? I want to be able to be there for you, and I want to let you know that it’s okay to lean on me or ask for help.”
You could also suggest that your triad talk about love languages or enneagram types or a similar system that can give language to how you three experience feelings, relationships, and needs. Think also about whether there is something you and your girlfriend can do to build more trust or intimacy in general, like spending more one on one time together talking about your hopes, experiences, and feelings.
It’s also possible that, for this specific person, your boyfriend’s comforting style is just more her speed. All people are different, and it doesn’t say anything negative about you that she often prefers his way of addressing her emotions. It doesn’t mean you’re bad, or inadequate, it just means that the idiosyncrasies of her low moods mesh better with the idiosyncrasies of his responses. Try not to take it so personally, and find ways where you two do mesh well.