The only other way is to cheat.
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You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
All if you have a gameshark
you can migrate him from emerald or sapphire.
You cant without action replay
Pokemon Emerald 386 is a hacked version of Pokemon Emerald that allows you to complete the pokedex without trading and/or hacking. It's a ROM that can be played on a VBA. You can get it at 4shared.com.