A little algebra can help us to find the difference (X) between BG3 and BG2. First, let's switch to a more common notation:
X = BG3 - BG2 --> X = (3 * BG) - (2 * BG)
Then some simple factoring will give us our answer:
X = (3-2) * BG X = (1) * BG X = BG
So, the difference between Baldur's Gate 3 and Baldur's Gate 2 is Baldur's Gate.
Except that, in BG 2, there was no Baldur's Gate?
Isn't that the point (my math is rusty)? If you subtract a game which doesn't have BG city from a game that does have BG city, BG city is the difference.