It looks like you have already watched a lot of good love triangle dramas (you have great taste ;-)) but I found some that are not on your Watch List.
Dal Ja's Spring- it's a little bit older kdrama that I found not long ago and I really like it, it's about woman in her 30s and her friends and collegues and how they face everyday problems, it's a romantic comedy with older women/younger man romance and well-written characters, love triangle included, Bonus : young Lee Min Ki plays ML!
Fated to Love You- I see that you have this drama on your PTW list and I encourage you to give it a try!, it's really cute and heartwarming drama with great cast, love triangle is a part of the plot and I must say that you are in danger of having second male lead syndrom!
Flower Boy Ramyun Shop- this romantic comedy is a little bit silly but I liked it so give it a try and you will see if you like it or not by yourself, it's also older woman/ younger man romance and it's funny so you will be laughing a lot!, love triangle here is really well done, I wanted her to be with both if that was possible xD
