Triangulation is a form of abuse, often associated with narcissistic abusers, which works in a way that gets the abuser what they want, by involving other people into the relationship. It will usually happen when you and the abuser are in some kind of conflict, and want different things; the abuser tells you what they want you to do, and you know you don't want this, so you say no, and stand your ground.
When the abuser realizes they can't persuade you, threaten you, force or pressure you into doing what they want, they go a roundabout way about it, by convincing other people to go and pressure you instead. Now how does that look like?
It can go multiple ways, they can come and cry to other people about how incredibly cruel, insensitive and selfish you are, not wanting to do this one thing to them, and how it's killing them, and ruining yours and everyone else's life. They'll often sprinkle in some lies about you, make you look cruel and twisted. They can come to your siblings, friends, relatives, family, and tell them something along the lines 'I know it's only right for them to do x, but they won't listen to me. However, they really value your opinion, and they might listen to you, please tell them to do x, it would be better for everyone.' This will make the person feel important and happy to be valued both by the abuser and you and to be given this important task of changing your mind, they'll become willing to do it. Or, if they go the crying and complaining route, the person will become annoyed that they're being vented on and feel like it's your fault they're now having to deal with this. The abuser might also add imagined disasters and catastrophes that might happen, if you do as you want, and not follow their idea of how things should go.
This person, having been manipulated into thinking they're doing something good, will go and try to tell you that you're wrong for wanting to do what you want, sometimes they'll accuse you of being selfish, yell at you, defend the abuser's idea, pressure you, accuse you of being responsible for abuser's "upset" by acting this way, and will wholeheartedly try to convince you that following your own free will is a horrible, disastrous idea.
And for you, this now means you have to consider, and reconsider over and over again, if you have the right to make your own call, because now everyone in your life seems to be on the abuser's side, and insisting that what you want is stupid, selfish, inconsiderate, hurtful, wrong, potentially disastrous. This, of course, is not true, but the abusers want you in that spot, where everyone you know is against you and on their side, convincing you to just do as you're told, or else. This can sometimes create intense pressure and feeling like you're all alone in the world, like nobody cares about you, and that any decision you make will be beaten down by everyone involved. It can also make you pull down your own choices after having to consider over and over if it's possible that this choice could be selfish or in any way hurtful, and this is not how people normally make calls, it's something you do under intense pressure and scrutiny, which is there only because the abuser wanted it.
These choices can be anything from where you go to school, work, where you live, how you dress, who you date, what you buy, how you respond to others, what you do, how you act. Often they'll be about whether you give the abuser what they want from you, and whether you agree to talk to them or not. The biggest triangulations I've experienced in my life were done when I've refused to talk to the abusers, then they felt it was necessary to lie and manipulate every person I knew to pressure me into extending contact - and all of those people have been lied to, and have been brought to a heavy emotional state in order to do a crazy thing like telling someone what to do. Normally people don't do such things, they understand it's not their place to dictate someone's personal decisions.
This is partly what makes the abusers so dangerous, not only they're willing to cross that line themselves, but they're capable of making others cross it too, making their every whim seeming like a life emergency where all rules of freedom and privacy are null. This also puts you in a position where you're forced to doubt yourself, your every decision, and even your senses, to figure out if you could possibly be right, when everyone you know is telling you that you're wrong. They want you to be in that desperate mindset, doubting your own senses, memories, decisions. It's almost like a form of gaslighting, that everyone participates in.
Your choice is, of course, never wrong. You always have the freedom to make your own decisions, even if the entire world decides to attack you for it and to tell you that you're wrong. The entire shitshow is happening only because the abuser thinks what they want is more important than your free will and your quality of life, and they're willing to lie and push people into doing their bidding, just to bypass your free will. You don't have to doubt any of these decisions, because any choice you make is right, as long as you're the one who made it.
Being trapped in that space where it feels like everyone is against you, and on the abuser's side can be extremely isolating, painful and emotionally heavy. It can put you in a state where you feel abandoned and like you're wrong and evil for every decision you try to make. However, this isn't true, all of the people who are telling you that you're wrong, actually have no clue what's going on, and have been fed lies. Not that it hurts any less that they've betrayed you despite not even understanding the situation! It hurts badly. They've been naive enough to fall for a manipulation, and their moral standing weak enough to try and infringe on your freedom, and you didn't deserve that. They've aligned themselves with a person who has hurt you and is attempting to control you, and that feels terrible, like you're alone and helpless against a whole horde of people.
One thing you can do is point out how it's not their place to make this call, and ask them what they've been told in order to come and tell you something as inappropriate and hurtful like this. I can't claim it's going to work every time, but some people were taken aback when they were informed of some of the stuff the abuser just 'forgot to mention' when sending them into action. Sometimes even that won't make them back down, because it's embarrassing to accept that you've been manipulated into being someone's tool, and they'd rather insist they're completely in the right. It's a messed up game where you essentially can't win, the best you can do is stick to your decision and inform everyone who comes to pressure you, that you're not in fact, doing anything illegal and if they want to change your decision they'll have to bring out an army to force you.
Triangulation goes hand-in-hand with scapegoating, and will make you feel like a scapegoat. Even if a lot of people will fall for the abuser's manipulations, not all will, and this is not something that you'll need to endure all of your life, especially if you manage to cut contact with people who are easily manipulated – you don't owe them a presence in their life if they're aligned with an abuser. There are people out there who cannot be manipulated easily, and will stick to their morals and refuse to infringe on your freedom, even when told a bunch of lies.
