Triangulation is when a narcissist introduces a third party into a relationship with the intention of creating insecurity, uncertainty, and jealousy in their partner.
Because control, domination, and power are the central aims of a narcissist, anything that leaves their victim feeling less than, insecure, or unsure of where they stand serves their ulterior motives. By pitting one person against another, the narcissist gains the upper hand and creates drama, competition, and confusion.
This can be done by bad-mouthing one person to another, spreading untruths, or sharing vicious rumors, for instance. The narcissist may tell their victim the other person agrees with their stance on something, thereby invalidating and creating self-doubt in the victim. They may unfavorably compare the victim with the other, increasing the victim’s sense of insecurity and inferiority. They may present themselves as the neutral all-knowing party in order to ensure they control the narrative.
To reiterate, everything the narcissist does is meant to ensure ongoing attention, admiration, control, and domination, i.e., never-ending narcissistic supply. Triangulation is just one more manipulative tactic, one more tool in their narcissistic abuse toolbox.