Things You'll Need
Instructions
Start both games on each Nintendo DS system, choosing a different starting Pokemon for each game. Play through both games until you reach Cherrygrove City. On the game you want to keep, make sure to catch a throwaway Pokemon, like Pidgey, on the way.
Enter the Pokecenter in Cherrygrove City. In the Union Center inside the Pokecenter there, join a union room and trade one starter for the throwaway Pokemon to gather both starters on one cartridge.
Reset the game from which you traded the second starter and start a new game. This time, pick the third starter and play through to Cherrygrove City. Have both games join a union room and trade the third starter to the main cartridge to gather all three starters in one game.
Play the first game until you reach Blackthorn City. Use your starters often, increase their happiness by not letting them faint in battle and raise their levels until they reach their final forms: 30 for Feraligtr, 32 for Meganium and 36 for Typhlosion.
Find the Move Tutor house and talk to the old man on the right. He will teach your starters Blast Burn, Frenzy Plant or Hydro Cannon depending on the starter's type.