If I'm with one friend, I'm fine. However the second another friend or other person comes into the equation I always feel left out.
I often feel as though my two friends, even if they don't know each other, are being a bit mickey taking about me and a bit 'yeah yeah, we'll humour you'.
It's just happened at school collection today. I was chatting away happily with friend A. Friend B came over. Friend A and B don't know each other but they immediately started to chat and left me out. I said something and Friend A burst out laughing as if I'd said something very stupid.
I always end up being left out in a group of 3. I went shopping with a friend recently and she brought another friend along without telling me, and they spent the day totally leaving me out.
Plenty of mothers don't allow their children to play in groups of 3. Someone is always left out or ganged up on or something. It's not just you. It's just a challenging dynamic - nature of the beast type thing.
Successful encounters of 3 require all 3 people to be mature and polite.