The classic three starter Pokémon from the original Pokémon Red and Blue versions are available in Pokémon Brilliant Diamond and Brilliant Pearl. You will be able to acquire these Pokémon in a specific method, and when you encounter them, you can add them to your collection. Here's what you need to do to find Bulbasaur, Charmander, and Squirtle in Pokémon Brilliant Diamond and Brilliant Pearl.
All three of these Pokémon can be found in the Grand Underground location. You can only find them in their respective regions. For example, you can find Bulbasaur in the Prairie Caves or the Sunlight Caves. You will need to find the specific biome for the three of them. Even then, encountering these Pokémon is not guaranteed.
You will only be able to encounter these Pokémon in the Great Dungeon after receiving the National PokéDex. After meeting all the Pokémon in the Sinnoh Region PokéDex, you will receive the National PokéDex, which means completing the game. You'll need to treat this as post-game content, so don't worry if you're exploring the Grand Underground earlier in your Pokémon Brilliant Diamond and Shining Pearl journey. The same goes for many of the Pokémon you wouldn't usually encounter in the Sinnoh region.