It outshone Pokémon Sword and Shield!
In the first 3 days of its release Pokémon Brilliant Diamond and Shining Pearl combined sold 1,39 million units in Japan. This includes the retail version, dual pack version and download card purchases. (And does not include digital downloads from the eShop.)
The record for biggest physical release for the Switch in Japan is held by Animal Crossing New Horizons, which sold 1,88 million units in its first 3 days of its release.
The release of Pokémon Brilliant Diamond and Shining Pearl beat that of Pokémon Sword and Shield, though it’s too soon to say if it’s sales figures will remain as strong.