It could only have been Fluttershy’s idea. Luna thought of it as well, but she prizes her friendship with Fluttershy far too much to risk suggesting anything herself. But Fluttershy knows Luna still has a thing for Twilight, and she’s always felt bad for her never getting her chance.
Before even talking to her own lovers or Luna about it, Fluttershy would bring it up with Amber. Given their occasionally tumultuous past, Fluttershy is wary of doing anything that might cause a rift between the two of them again. But Amber gives her okay to the idea, so they begin plans to get Twilight and Luna together.
Fluttershy tries to simply talk about it with Twilight, but Twilight gets the wrong idea. She thinks Fluttershy means that she herself is interested in Luna, and takes some time to think about it. When she does, she tells Fluttershy it sounds like a perfect idea. Dinner plans get made, but both of them expect the other will be going. It’s not until the day of that they realize their mistake, and Sunset practically dies laughing.
Not wanting to disappoint their friend, Twilight goes through with meeting her at the restaurant. She has every intention of explaining the miscommunication, and that she’s flattered but completely uninterested in dating anypony else. Once they’re face to face, however, Twilight worries about hurting Luna’s feelings and stumbles through every attempt to bring it up. Luna interprets this as general nervousness at being on a first date and does what she can to make Twilight feel comfortable. And, much to Twilight’s dismay, she’s absolutely charming.
Halfway through the dinner, Twilight abandons all pretense of calling it off. By the end of the date, she gladly accepts a second. She returns home with a spring in her step, which doesn’t exactly go unnoticed. Sunset just grins knowingly from the second Twilight walks through the door.
Everything about the relationship fills Twilight with equal parts euphoria and anxiety. She loves her new marefriend, but she’s more than a little concerned about being the only one who’s dating a third pony. One night while Twilight’s out with Luna, Sunset’s complaining about Twi’s mood swings to Amber, who had accompanied Luna to Ponyville in order to catch up with her friends. Amber jokingly suggests that the two of them start dating so Twilight won’t have anything to feel bad about, and Sunset jokingly agrees. Fluttershy suggests they actually do it, just to see how things go. Worst case scenario, they might actually succeed in getting Twilight to stop worrying about things. The two of them figure ‘what the hell’ and go for it, just for fun. They jokingly go out on a date, and even share a joking goodnight kiss. They do it again on Twilight and Luna’s next date. Nopony’s really sure when exactly they stopped ‘pretending’ to date and wound up actually dating, including them.
Throughout everything, Fluttershy remains interested in only being with Twilight and Sunset, at least in any official form. She had been very close friends with Luna for years, and the two of them have always been oddly affectionate. Once Sunset and Amber began dating, that same tendency quickly spread to Amber as well. She never grows to love them like she loves Twilight and Sunset, but the nature of her friendships with both of them causes a lot of confusion to any onlookers.