I think love quadrilateral (three people being in love with one person) is very rare unless it's like a harem/historical situation.
Unless you mean it in a way that four people have to be involved and be in love with eachother and not necessarily one person having three people that are in-love with them. If this also counts, i would say the Worst of Evil (kdrama)
5. Marriage, Not Dating 2ML broke up with FL. 1ML went into a contract relationship with her to avoid blind dates. FL started dating 3ML. 2ML wanted her back.
6. The Worst of Evil (already mentioned by marblebees but it wasn’t counted) - ML and FL are married. 2ML likes FL and they date bc undercover operation. 2FL likes ML and they date bc undercover operation
7. The Romance of Tiger and Rose - FL and ML are married. 2ML likes FL. 2FL likes ML. FL and 2FL are sisters
10.Twinkling Watermelon- FL 1 liked by ML1 & ML2. ML 2 liked by FL 2. FL2 &ML2 date. FL1 &FL2 date.
10 Dramas/Movies where the lead character starts off being the new kid on the block or new in town.(i.e. they begin the story at a new school, community, place, situation etc)