This is sooooo rare and I don't understand WHY. I've also asked this before, and I pretty much got the same suggestions because, well... I suppose this plot isn't that popular. :/

I believe it's more common in Taiwanese dramas, but it's been so long since I've watched them that I can't remember many titles beside the TW version of Fated to Love You (in the Korean version, he made his decision way too early on, but I suppose it's because the characters in this version are a bit older/more mature. )

Heartstrings is a Korean one... though it's almost too obvious. And it isn't very romantic, and not sure if this is a spoiler, but the kissing scenes? They're awful.

I really can't think of anything else, besides a few that have already been listed here. Blah. I'm always on the search for this type of triangle.