How do you get the three on Pokemon diamond?

The steps below explain the process of getting the nationaldex

1. Defeat the Elite Four and Champion Cynthia.

2. After the credits roll, go back to your starting town (Twinleaf Town if you chose Dawn, Sandgem Town if you chose Lucas)

3.Talk to your mother.

4.You will receive the National Pokédex

Note:

- You can talk to Prof. Rowan for the Sinnoh Pokédex

- After receiving the National Dex, you can travel to Kanto, Johto, Hoen and Sinnoh