Are you the one that always walks behind your two friends on the sidewalk? Or the one that is always in the backseat, even if you get picked up first? Well, that is what I use to be--the third wheel of a friendship.
My two best friends, since before kindergarten, I guess were never really my friends in their eyes. In mine however, they were the best people in the world. That was before they randomly stopped talking to me after we graduated high school. One moved away and the other one I still work with so she can't really not see me. Everyone we knew believed we would never leave each other behind- I thought that too.
Being the third wheel in the friendship didn't hit me until they stopped inviting me to do things with them and started doing everything as just the two of them. So take a second to figure out if you are also the third wheel in the friendships you have and if you aren't well then think of who might be in your group of friends.
If you are, you can try to change that and make it known. Talk to your friends about your concerns, even though that might be hard because you don't want to sound whiney but it will help. Another way to not be left out is come up with ideas that you all could enjoy, even though you will probably have zero control of the radio when you are sitting in the back.
Just make sure, you are never, ever being taken advantage of!