No one is bad. FL loves ML. ML genuinely loves another SFL. SFL may either love him back or is with another person. sfl can have feelings for ml even if she is with another man. but no one is bad. no cheating. sfl is not a crazy b-xch. its just unrequited love from FL's side. ML is a good guy, FL is a good gal, and so is SFL. just one sided love, which may or maynot turn into true love

 Ever Night (season 1) - ML and FL are very  close with each other since childhood (in fact form sth like surrogate family)  and the ML  honestly falls for another girl ( who is a  sympathetic character , suits him too and reciprocates his feelings).  All the characters are pretty  young, inexperienced  and only learning their own emotions while growing up and maturing.  The triangle here is not about who is better  but about  recognizing which feelings  and connections run deeper. 

Handsome Sibilings -  with that one I have problem with saying who is ML , SML , FL, SFL though. There are 4 characters also one girl gets a bit more screentime and comes earlier in the game ( let's call her FL) . So FL  def  fancies ML, ML maybe fancies her a bit  ( or  maybe not that much, probably it's unclear even for him;))  but he doesn't want any romantic entanglements. Later on he falls  for another girl ( and FL moves on with other guy)