You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
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The only other way is to cheat.
You can't catch wild Treeko in Pokemon Emerald. You need to choose it out of the three Starter Pokemon that are offered to you at the beginning of the game: Treeko, Torchic, and Mudkip. to see it, battle rival or other trainer
Pokemon Emerald Enhanced is a hack of Pokemon emerald. The most special thing about it is that all Pokemon are available somewhere in the game. No trading or gamesharking is necessary. Toward this end, a few new areas have been added as well. As a final note, the three starter Pokemon for this hack are Zapdos, Articuno, and Moltres.
The three legendary Pokemon in Pokemon emerald is Kyorge, Groundon and Rayquazer.
You cannot without a cheating device such as the Action Replay. Sorry.