The common shapes it takes
Bringing up an ex's qualities as a comparison, venting about you to mutual friends before talking to you directly, or quoting 'what my mother thinks' mid-argument are all versions of routing a direct issue through someone who isn't actually part of it.
Why it's effective at creating insecurity
Comparison and an audience both add stakes that a private, direct conversation wouldn't have — it's harder to stay calm about an issue once it feels like other people are watching or judging the outcome too.
Why people do it even when it backfires on trust
Routing conflict through a third person avoids the vulnerability of a direct ask or a direct complaint — it can feel safer to let someone else's opinion do the confronting than to simply say what you want.
How to redirect it back to the actual two people
Naming it plainly — 'this is between us, I'd rather we talk about it directly' — refuses to let a third party carry weight that was never theirs, and usually exposes pretty quickly whether the real issue was ever about that third person at all.
Occasionally wanting outside perspective isn't the same as triangulation — the difference is whether a third person is being used as a tool against you, or genuinely consulted as support. Patterns matter more than any single instance.
Recognizing the pattern in general helps. Naming who's actually being routed into your conflicts lately is the harder, more useful thing to look at. Name that person, in OLENRI