Bringing another girl in.

So my boyfriend introduced me to his girl best friend and she is super hot. I’ve never been one to say girls are hot but she is. So we were talking and I was like I wanna three sum. I’ve never kissed another girl or done anything like it. So I asked them both and they both were like yeah and I’m still for it but I wonder if he will like her better in bed and I’m so worried I’m not good enough. I love him so much and I know he would never leave but I worry. Has anyone ever felt this way? I’m so lost right now.