I don't think that one instant of finding you haven't been invited people should immediately go to "cut them off" etc.
As I say to my dc, friendship is about give and take, and also most people don't intend to be mean, so best start out by assuming it was a mistake/accident/badly done etc.
If you always leap straight to the worst thought, are you prepared for someone to do that for you too?
Op doesn't know whether it was a spontaneous gathering, one where two of them said "oh let's go and see" third was passing, saw the cars etc.
If they've just moved house maybe the others came to help shift, for example.
Or maybe there was something came up and they needed to talk about it - I was round at someone's house a couple of weeks ago to help them fill out power of attorney letter - thy needed two witnesses.
If you take it as the op should immediately take offense, then maybe the others should take offense that op would have seen the cars outside and didn't pop in to see them - clearly she didn't want to see them or she'd have knocked.
Op if you like them, then you would be silly to take a single event and cut them off without thought. If you normally are invited, and you get on well when all together then enjoy a friendship that has lasted 14 years.