Get a Action Replay DS or trading.
You have to trade at the GTS
There is no other way.
you cant in platinum but you could trade from Diamond and Pearl or another Platinum.
Action Replay
You cant Actually, you can. Just do one of these: 1. Trade on the GTS 2. Ask your friend to breed theirs, and give you the egg or start your adventure get your first starter trade it and do this one more time to get all the starter Pokemon
type this code 010075DA and now u can get all three starter Pokemon
no you can not
Trade
Trade for them.
you must trade with a freind that has a starter pokemon that you dont have.
Only on Pokemon yellow.
i think you go to pro rowern and battle him p.s he has all the starter Pokemon and they are all lvl 100