Doesn't quite fit but ...
Lovers
Hapimari: Happy Marriage
Fated to Love You (you can watch both the Taiwanese & K-versions if you like)
Loving, Never Forgetting
all had situations where a 3rd wheel tried to come between the couple who are already an item, but temporarily separate due to complications in their relationship. The 3rd wheel takes advantage of the situation & tries their best to make their case to the female lead. These guys aren't necessarily evil assholes though, they just want the heroine for themselves. That's why these recs aren't a perfect fit for what you're looking for, but close enough I think.