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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 go to professor britch and he gives the fire red starters
All Pokemon from Kanto, Johto, and Hoenn can be obtained in Emerald. Susrskit, Masquerain, Meditite, Medicham, Roselia, Zangoose, and Lunatone must be traded from Ruby or Sapphire. Other Pokemon like all of the starters must also be traded over.
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
By trading the other starters with other players of Diamond or Pearl.