Slifer the Sky Dragon
Slifer can be obtained only by unlocking the Golden Millennium Puzzle Booster Pack. To unlock the Golden Millennium Puzzle Booster Pack, beat Simon and then, through game play, the card will turn up.
Obelisk the Tormentor
Unlock the Green Millennium Puzzle by beating stage four 10 times. After that, regular game play will unlock Obelisk.
The Winged Dragon of Ra
The Ra card was a part of the Monthly Yu-Gi-Oh! Magazine. The password is DE893EA8BF7621A3.