Hiev:
You should try watching When I Was The Most Beautiful. It’s currently airing. The FL is in a love triangle with 2 brothers.
Small Spolier Warning:
Within the first 4 episodes she marries the older brother despite the younger brother being in love with her. But, something happens to the older brother which leads to the younger brother trying to pursue the FL again.
Thank you a lot. this is what I am looking for... Mature Kdrama with the theme of a complicated relationship, threesome relationship and infidelity. Please let me know if you have more recommendations (Kdrama).