There are various game save editors within the internet, and action replay of course. Another popular way is to trade within games. Sadly there is not "legit" way.
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You can first collect all of the Pokemon for the hoen dex in Pokemon emerald you will get 1 of the 3 Pokemon gold/silver/crystle series starters then use the get all of the starters trick to get them all then migrate them to Pokemon diamond or pearl.
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i assume you mean the 3 starters?? by trades
They are actually all 12 starters. You can only choose 3 starters.
to show a challenge but it is easy to get more starters
trade with your nerd herd
There's only one way to get all 3 starters in Pokemon HeartGold/SoulSilver, but you will need to have 2 DS's and 2 games. You start with a Pokemon and trade it on your other game on your other DS. You start a new game and then do the same with your other 2 and then trade them back. You then have all 3 starters.
cheats
The only way to do that is to trade with other people.
so here is what i did. i got one starter from ruby and i gave the starter over to my Pokemon Colosseum game. next i made a new game and got a new starter etc. then when i had all 3 in Colosseum, i gave them to fire red. but if you think ur profile on ruby/sapphire/emerald is too rare to give up for all starters u can just give the really good pokemon, either give them to a different game of just dont do it
the only way is to trade from Pokemon sapphire or ruby.
gameshark