I spend quite a bit of time looking for it and I can't find it. Starting to wonder if the author deleted it. Its a polyamory fic with three ponies. Two are already in a relationship at the start, and number three is hanging out with the other two a lot. They are all mane six and I want to say that Applejack and Twilight are together and Rainbow is the one hanging out with them a lot. I am not 100% sure on this though. Could be Applejack was the one who hangs out with the other two. Small chance one or more of the ponies might be someone else from the mane six. The rest of the mane six are asking questions about the relationship between all three, which leads to the three starting to ask questions themselves and ultimately deciding to try polyamory.

Does any of this ring a bell with anyone?

https://www.fimfiction.net/story/112508/just-a-couple-of-ponies

Maybe this story?

5501626

That's the one! Thanks a lot :)

Also understand why I couldn't find it. I could have sworn it had finished, so I had the "complete" filter on whilst searching.

5501651 glad to have helped.