you could use action replay or trade with friends
you can start over
Get a Action Replay DS or trading.
sorry but you have to get another diamond/pearl/platinum game and trade the other starters from there.
go to all three lakes and you shall find all three
You have to trade at the GTS
No, because charmander was one of the original three in all the rest of the games so they didnt put him in Pokemon diamond.
Catch the Pokemon or head straight to Sunnyshore City
There is no other way.
If you have another copy of Pokemon Diamond or have Pearl or Platinum with a different DS, you can trade between systems and have one of them start a new file and pick the Starter you are missing to trade to your primary Pokemon Diamond file. It'd be easier to have a friend help you as well.
You delete all of your progress and start over
It's impossible -- you must trade to get all three.
you have to catch all of the pokemon in diamond or SEE all of the pokemon