Defeat all the opponents at Realgam's Colosseum to unlock Trade Machine to transfer Pokemon to and from Pokemon Ruby and Sapphire.

Successfully complete the game then go to the Pokemon Center in Pheanic town. Go downstairs and talk to the woman. You will now able to transfer Pokemon from your game to Pokemon Ruby and Sapphire and vice-versa.

Defeat the evil mayor in the tower after Nascour, then go to the Trading Center in
Phenac at the Pokecenter. Talk to the lady, then connect a Game Boy Advance with Pokemon Ruby or Sapphire. Trade the Pokemon you purified. Pokemon from Pokemon Crystal, such as the three legendary dogs can be traded to your Game Boy Advance. However, the Pokemon from Pokemon Crystal do not go in your Pokedex.

Use the following trick to transfer Pokemon from Pokemon Ruby or Sapphire to Pokemon Colosseum, without having to lose a purified Pokemon. While playing Pokemon Ruby or Sapphire, look for a Nincada, and catch it. Transfer the Nincada to Pokemon Colosseum. Leave Nincada at the top of your party, holding EXP Share. Make sure you have a strong Pokemon to back up Nincada so that it does not faint without gaining experience. Also leave an empty slot in your party. Then,
battle with Nincada until it evolves (level 20). If you had an empty slot in your party and also a spare Pokeball, not only will your Nincada have evolved into Ninjask, but you will have Shedinja in your party as well. This means you made one Pokemon become two. Then, go back into the Trading Machine, and trade Ninjask back to the game, recovering the Pokemon originally traded Nincada for. Use the freshly "created" Shedinja and trade it for another Pokemon of your choice, such as Kyogre or Groundon, or even one of the three starters. Note: You can use this trick to get all three starters and more than one Celebi (if you have the Bonus Disc), but it requires you to start a new game in order to get a different starter.