One of the girls is a womanizer, but only in the beginning. Tldr she used to seducing women to further her own goals. After the female leads hook up she stopped doing this.
Altough one of her previous sex partners loves her (she is the 3rd girl mentioned in the summary), there is no real love triangle.
Well, one of them has a husband, but it's not a real marriage, they are just married in name. To make it more difficult, her husband loves the other girl. But that's a minor point in the story imo. Still not a love triangle, but some sweet yuri cucking there.
Well, in the start the author tries to establish the girl as the mistress of the married girl, hence the title, I guess. But it is a huge stretch, don't believe that. The title is no more than a clickbait.
Summary is misleading too. The "third girl" is just a minor character, basically has nothing to do with the story, so I don't really get why the summary is like that.