Im a third of my first ever successful 3-way friendship. All of us are mid/late thirties 2/3 of us have known each other 15 years and we met our 3rd amigo about 8 years ago. We have a very easy, happy friendship and endeavour never to leave one out. Thats not to say we don't organise stuff with just 2 out of the three, but none of us would ever want one to feel left out. I am by far the softest and easiest target in life, and even I can cope. There are times Im sad about stuff, but that's my issue rather than something anyone has done.
They sound as though they need to do a lot of growing up, and maybe its worth leaving them to it for a while. I am sorry to hear you've had a shit year and people can be so mean about party invites as adults.
To put it into perspective one of my best school friends married my ex boyfriend and I was bridesmaid. Didn't think anything of it, I am married with 3 kids, and zero harboured feelings for any of my exes, and I am truly happy they are happy together.
Going to go with your friend probably wouldn't invite me either 
It is so hard when you feel left out, can you do a trip with some nicer folks at some point? @Littleteacup1
You deserve better I would say 
H x