This theme appears much less than 2 guys 1 girl. Although most dramas do have some element of a triangle.
Here is a short list:
Precious Time - 1998/Japan The triangle is almost nonexistent. . . BUT it is there! Also, the drama is great. I gave it a 10!
All About Eve - 2000 /Korea I cannot remember everything, but Kim So Yeon's character competes for everything! I rated it as 7 if that helps.
My Beloved Sister - 2006/Korea
Golden Bride - 2007/Korea
Cinderella's Sister - 2010/Korea Many people did not like this one; however, I thought it was pretty good. The acting was great! Although I am not a huge fan of Moon Geun Young.
Legend of Concubine Zhen Huan - 2012/China/Historical I enjoyed this drama so much! There are concubines so there are multiple triangles. However, this one is not so much about romance, as it is about the how life within the back palace warps the minds of the women living there. REALLY WONDERFUL!
Wild Romance - 2012/Korea This is probably the only love triangle with comedy, that I will recommend. IT IS SO FUNNY! Honestly, the story gets messy, but it definitely compensates with humor, acting, and chemistry. Please try it if you haven't already!
Legend of Lu Zhen - 2013/China /Historical
Babysitter - 2016/Korea 'Babysitter' is not really a LOVE triangle, but it does involve two women and one man. By the way, it is very INTENSE, for four episodes.
Tribes and Empires: Storm of Prophecy - 2017/China/Historical Um. . . This is definitely a must! Although there is not a large focus on romance and there are multiple storylines, there is a love triangle, and it gets dark. This drama has some of the best actors!
Almost all historical Chinese dramas, revolving around romance, have the multiple girls 1 guy triangle.
I hope this list is helpful. :)