Technically, yes you can get all the Johto starters... mainly one from the Emerald versions if you have completed 202 entries of the Hoenn dex, Birch will give you a "surprise" and you can choose one of the Johto starters. But that's one way to get them, though that'd take quite some time to achieve that... But to get the other two, you might have to trade with your pals or from Colosseum/Gale of Darkness.
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
The only other way is to cheat.
No. No you can't. You must trade or start over.
Trading.
trade from another Pokemon emerald game.it is the only chance to obain another starter without action reply
a bunch
In Pokemon Emerald, Eevee and Poliwhirl can only be obtained via trading...
All if you have a gameshark
you can migrate him from emerald or sapphire.
You cant without action replay
You can't unless you picked it as starter pokemon.
its simply not possible