It's not possible to do that. You cannot get the starters from johto,kanto or hoenn unless you get them by trading if you got them on Pokemon diamond/pearl or insert the gba game in the slot if you got them when you get to pal park, or theres just the way of cheating which I never do.
OMG there is a way jeez k look if u have emerald catch all 200 Pokemon from the hoenn pokedex and once ur done than what ya gotta do is talk to birch and he'll give u a choice of ONE so choose careully than u ovibously go to palpark blah blah blah -BillyDuong-PEACE
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The three Jhoto starters are Chickorita (grass-type), Cyndaquil (fire-type), and Totodile (water-type).
Baltoy koffing and paras
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
The three starters in Pokemon Crystal are Cyndaquil (the fire type) , Totodile (water) and Chikorita (grass ).
You don't not unless you trade or cheat