No.
To obtain all three starters you need to trade with someone.
in my heart
Get a friend to trade them to you.
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
Trade with other people who have heart gold or soul silver.
By cheating or trading with other people.
you cant
Get Pokemon emerald and then you can get all three without cheats or trading
By trading the other starters with other players of Diamond or Pearl.
the starters the three dogs ho-oh lugia celebi that's all i can think of
two words: YOU HACK
You can get three starters from different areas by completing the game (one at the beginning from johto, and after defeating red one from kanto and one from hoenn) but unless you trade or cheat there is no way to get all three starters from any one area.