first what came to mind The Innocent Man SFL is kind of bad person, as character/screen time she is equal to FL. SFL was love of his life, she betrayed him and drama is about his revenge, though his feelings are still hot, love/hate
Queen of Ambition - kind of same story, FL and her story, relationship to ML is like SFL from innocent man, though that FL is worst )))) also here is SFL who is more like FL (I mean good person and her story is like FL from innocent man)
overall these 2 dramas have many many common things and similarities, but FL/SFL are replaced
in The K2 I would say SFL is real FL, more screen time, more depth, very strong, interesting, not good person though it's also not that simple, she is kind of person who had strength from being victim to become hunter and most big-strong hinter . most of the fans (me too) think she had more chemistry with ML then FL
Ja Myung Go historical drama, FL and SFL are sisters and both characters are developed, SFL point of view, her relationship to ML is quite deep, not typical dynamic between ML-FL-SFL
Tempted also not typical dynamic between ML-FL-SFL, also SFL point of view, her relationship to ML is quite deep, I would say SFL is more complex and interesting character, also sometimes bad girl.
*** I didn't mentioned dramas with multiple leads, I mean where ML-FL-SFL isn't case and SFL has her own story/man. also I didn't mentioned dramas with not so developed SFL character
p.s. there are more and interesting dramas where SML is developed and/or FL falls for him, if you interested love triangles like that you can see this list:
https://mydramalist.com/list/3rWoKqA4