Basically whenever the story sets up a love triangle/corner or similar in the story with the implicit expectation of having to choose between the partners but then the story pulls the rug under you asking "Why can't they all be happy?" and allowing all involved character to date each other
Rentarou x Hahari x Akane - The 100 Girlfriends who Really, Really, Really, Really, Really love you: Rentarou is a guy who falls in love easily to the point that, at the age of 16, he has confessed to 100 different girls and has recieved 100 rejections (With his first one being at 8 months of age), he's then confronted by the God of Love who confesses that, due to an accident while watching "Castle in the sky" by Studio Ghibli, he accidentally made Rentarou have 100 soulmates. He explains to Rentarou that having 1 soulmate is an incredibly lucky occurance, to the point that those people who are blessed to have one have exhausted all their luck in life and are prone to accidents that scale in severity until they meet an early end. Thankfully their luck resets if those soulmates find each other and live together. This still applies to Rentarou and he only technically needs to date 1 soulmate to avoid death, but it also means that the 99 other soulmates are fated to die without him. And he's instantly confronted by having Akane (The tsundere archetype) and Hahari (The "Lewd" Archetype) fall in love at the same time with him. He considers dating them in secret, but he genuinely loves the two of them and can't bring himself to do something that would hurt them in the future, so instead chooses to come clean about his feeling and being unable to choose, so instead he confesses his love to them and asks them to date him at the same time. They have some hang ups at first, but eventually decide to give it a test. At this point in the manga, the two girls have even develop feelings with each other, being one of the few girls to actually love someone as strongly as they love Rentarou
UNDEADXUNLUCK - Rip x Leila x Latla: Rip and Latla are both members of the UNDER organization that are seeking to steal an artefact from "The Union" that, theoretically will allow them to time travel. They want to time time travel to save Latla Sister and Rip's fiancee Leila, who tragically died due to Rip's Negator ability, "UNrepair", and Latlas "UNtruth" which manifested during a life threatening operation that Rip was performing. They eventually would die in battle against UNdead. Then, when Fuuko is entrust with the final loop, she makes sure to guarantee everyone's happy ending, which among other things, involves studying over 100 years to learn how to properly operate someone's heart. Later, after the fight everyone assumes, including Latla, that Rip is going to have his happy ever after with Leila while Latla will be sideline. Instead, Rip confesses to Fuuko that he loves them both equally, he has always done so. And that he couldn't care less what other people think, since it's their happiness and not the strangers, So in the day of Leila & Rip's wedding he proclaims that the three of them will get married to the surprise of the guests and Latla herself.