You have to trade. Borrow a friends DS and put a Pokemon game in each DS. Then get your starter Pokemon trade it over to your other game then restart your new game and keep doing it over and over again. Good luck :)
Chat with our AI personalities
By cheating or trading with other people.
Trade with other people who have heart gold or soul silver.
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
You don't not unless you trade or cheat
By trading the other starters with other players of Diamond or Pearl.