The only way to obtain all three starter Pokemon is through a trade. The GTS building is in Jubilife City, and you need to have WiFi Connection set up before you can trade.
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Do you wanna know how to get them?
You start out with one, you have two friends start with the others, then trade.
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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.
The only way to do that is to trade with other people.
trade with friends that also have a ds or buy action replay from a local game stop.
You can not catch any of them in black or white, but I think that you can catch all 3 in diamond.