(FYI, this is not canon to the prime universe, or canon universe, where all three tribes were probably separated from each other for several decades or centuries. This is my alternate universe, which is a crossover verse)

In this universe, all three pony tribes were together, up until a few years before Sunny was born. So here, Queen Haven (At this point, a Princess so just Haven), Phyllis Cloverleaf, Argyle Starshine, and Alphabittle Blossomforth together at a park. They knew each other when young and possibly friends, but not like Sunny with her friendships, but the races were together. They were probably just teens here, so a little younger than our main characters in the movie. But this here is why Argyle years later when he was middle aged wanted to bring every tribe back together and Sunny fulfilled the thing Argyle wanted to do.