As far as I know there is not other way besides using wi-fi or trading.
catch all pokemon
action replay or trading
By the GTS or trading with your friends
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
Get a Action Replay DS or trading.
its simply not possible
It is impossible to get all three starter Pokemon in one game without trading, unless you are willing to use an Action Replay.
You'll need an Action Replay for that, or trading restarting trading restating trading back
You don't.
trading with people who also own a copy of Pokemon
trade with friends and/or wifi trading
There is no other way.