The lying to cover their own backside has included the "manufacturing" of documents to "back" up what they are saying & prove that I'm "making things up". They have an advantage in that there are two others doing the same thing. I refused to get involved in what they were doing, so I became a threat in their eyes.
The thing that annoys me is that they were caught lying to me & denied doing so. But 15 years of being willing to trust them, were thrown out the window when they were caught out.
The backstabbing & their activities cost me my job, due to not joining them.
That last line changes everything. You're saying you've lost your job? That's terrible. But it does entirely change the context of your question.
When you say 'What do you do...?', the first response to that question has to be another: 'What do you want?' Seems like 'the damage has been done'. Are you after revenge, or exoneration, or what? Do you want to clear your name, with a view to getting a reference? What?
It seems to me that you need to take a step back, and try, however hard it is, not to let your entirely understandable feelings of resentment and disappointment cloud your judgement. Given that you are, now, rightly or wrongly, in whatever situation you're in, what's the situation you'd
like to be in? And what's the best thing you can do to bring that about? However galling, 'writing it off' and putting it down to experience might be at least part of the answer.
Horrible thing to happen. Try not to let the bastards grind you down. However this ends, they're bastards and you're not. That's worth more than any job, when all's said & done.