If Margot Robbie's Harley Quinn returns in Suicide Squad 3 or elsewhere, James Gunn and Peter Safran's DC Universe should avoid rehashing her Joker origin story. Ever since she was introduced to the DC canon, Harley Quinn's story has been interwoven with the Joker. As depicted in both David Ayer's Suicide Squad and the opening of Cathy Yan's Birds of Prey, Harley starts off as Dr. Harleen Quinzel, until she falls in love with the Joker and falls into a vat of chemicals, becoming his partner in crime Harley Quinn.
https://screenrant.com/dc-universe-harley-quinn-joker-origin/