That's impossible.
Chat with our AI personalities
It is impossible without trading from Diamond, Pearl, or Platinum, or without the use of a glitch or cheating device.
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
No, you can only get all starters without trading in Pokemon yellow, but Leaf Green is a newer version of Pokemon green, so it's impossible.
By trading the other starters with other players of Diamond or Pearl.
Try the GTS trick.