I'm gonna be very sad when my girlfriend hangs out with her friends and not me.
Does she hang out with her friends often?
Yes, she usually does. I'm tired of always being the third wheel.
It seems that you want more attention from your girlfriend. You should talk to your girlfriend about the time you both spend together and let her know that you would like to hangout with just her more often. This will help her understand how you feel.
Thanks for the advice. I'm still going to be sad.
There's nothing wrong with being sad, but I hope you take my advice. Your girlfriend may not realize how you are feeling and that her behavior is impacting you so much. Telling her that you want to spend more time with her is fine since it's normal to want the attention of your partner.