The fact that D3 outsold D1 and D2 combined suggests that while it might have been disappointing to some subset of players, clearly they managed to find a much bigger audience that enjoyed the game nonetheless.
You are still comparing sales to game engagement and longevity.
You also need to shrink the amount of D3 sales to compare it to D1/D2 sales because the game market has grown wildly since they released. Then adjust that because it was a sequel a decade in the making.
It's like comparing the star wars sequels to the original films. The nominal values are way bigger, but if you adjust them for inflation they might tell a different story.
