No one here knows you, so maybe there is a reason. Maybe you're truly awful - nothing you've said sounds truly awful, but let's go along with it for the sake of this.
If you were friends with someone who was truly awful, would you not just distance yourself and let the friendship die? Most people wouldn't string someone along the way your friends have, even someone truly awful. What's the point?
Likewise, most people wouldn't exclude someone and then deliberately talk about how they've excluded them to their face - even if they were awful. Again, what's the point in that?
It seems to me like they've identified you as 'weak' in some way, and they enjoy putting you down (is there a 'ringleader' between the three of them at all?). This is nasty behaviour, regardless of anything you may have done wrong.
So the only conclusion is that regardless of any feelings they may have about you, they are being horrible. I urge you to mentally draw a line under this friendship - don't cause any drama, be polite and kind (for your own sake, not theirs). But I would just be busy whenever they want to meet, and start tapering off replying until they give up. Just let it die.
I imagine they're quite insecure and unhappy beneath it all. Leave them to it and move on! I've been through something similar, I know how it knocks your confidence. But I've no shortage of friends now - something I can't say for some of the other girls involved 