First off, that sucks... I can only imagine what's going through your head right now.
My advise, like a post above would be to invite them all to a night out for some beer, wings, whatever you're into. Stay calm, behave like its no big deal and that you're genuinely interested in how their trip went. Then once they figure you're ok with it, gently ask why they didn't ask you.. (if it hasn't come up already by one of them). If you start getting lame answers drink your fill of beer, excuse yourself to the washroom and leave. Leaving the bill with them. That should send a message and if any or all come back with an apology or whatever, then you can decide what to do with regards to the relationship.
Sometimes relationships like this run their course and it maybe time to move on.
Good luck, hope it turns out well for you.