How to meet with a friend and her guy without feeling like a third wheel?

I have a friend coming to visit the town where I live this weekend. She’s asked me to have lunch or dinner with her and her guy, and of course I said yes. She is very sweet, kind, caring, etc.; I have yet to meet her guy. We’ve mostly become “long-distance” friends; we email frequently and try to stay friendly.

Do you have any ideas for minimizing the “third wheel” feeling for when I do lunch/dinner? Also, I tend to be anxious around new people. Any ideas for minimizing this feeling?

Finally, I’m (mildly) concerned that my friend will worry about her guy and me…she’s stated on a number of occasions that she feels we have similar interests and are similar people. I have no interest in taking another person’s guy, but should I dress “down” anyway? (I have no experience with what etiquette dictates here; most of my other friends were late bloomers or didn’t invite me out with their SOs.)

Thank you for any and all help you can provide! (Does Miss Manners cover this sort of thing??)