I'm currently dating three people, all men, herein X,Y and Z.

X and I have both dated A, A and I dated B (who also had sex with X).
X, Y and I all have relationships with C. X and C have a relationship that is very low key, sporadic, and sexual. C and I have a relationship that is emotionally intimate and sometimes sexual. Y and C have a relationship that is very similar to the relationship that Y and I.
I am dating Z. Y and Z previously had sexually intimate, emotionally intimate, co-habitting romantic relationships with D.

None of that creates a polygon.