In Pokemon platinum how do you get charsard venusor and blastois?

Answer 1

You have to migrate from Leaf Green or Fire Red, or trade with a friend or by using GTS. Or, if you have one on Sapphire, Ruby or Emrald because of trading, you can also migrate through that,Or Just Use The Action Replay Pokemon Modifier.

Answer 2

after you get national pokedex your able to migrate from any game.