finish the nation pokedex
Absolutely Nothing.
Nothing significant happens if you beat the Elite 4 three times in Pokemon Diamond, unless you include acquiring huge amounts of money and leveling up your Pokemon a very good amount.
After 20 wins, your rival's Pokemon level up.
Now you can capture some legendary Pokemon like Giratina and Heatran.
-mongmong908
No, you do not have to beat the Elite 4 to get the National Dex in Pokémon Diamond.
Beat Cynthia after Elite Four
After you beat the elite four.
the credits start
Beat all the elite four and there champion
Then you beat them:)
catch legendary Pokemon like giratina and heatran