The only way to do that is to trade with other people.
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po
There's only one way to get all 3 starters in Pokemon HeartGold/SoulSilver, but you will need to have 2 DS's and 2 games. You start with a Pokemon and trade it on your other game on your other DS. You start a new game and then do the same with your other 2 and then trade them back. You then have all 3 starters.
gameshark
you can not get all 3 starters unless you trade yea u can u just need lots of masterballs or a code to let u take other Pokemon
No, you will not get all 3 Starters in Pokémon Black 2, you will only be allowed to choose 1 Starter out of Tepig, Snivy and Oshawott and you will need to trade in order to get the other 2 Starters.