While we now know that Rebecca is the mysterious "her" in the flash-forward, we don't exactly know why everyone is going to visit her. There have been a few theories thrown around — like brain cancer or a heart attack — but we really believe she has dementia. Not only have there been subtle clues throughout the season — like when Jack makes a comment about how they will be old and grey together and not remember what they ate for breakfast — but it also could explain Beth's pin-the-tail-on-the-donkey game.
The Big Three used to play that game every year during their joint birthday parties, so it's possible they are reuniting on their birthday in the flash-forward to play the game with their mom. If Rebecca does have dementia, this could be the Big Three's way of re-creating those memories with their mom one last time before her death. Perhaps the whole show has been the Big Three reminding their mother of their family's story — it would literally be the Big Three telling Rebecca, "This is us." If that's the case, the season three finale could end with Rebecca dying, and the following seasons would piece together how everything got to that point.