How do you get shaymin, arceus and darkrai?

Answer 1

All of these pokemon are obtained legally by using the key items you receive at a Nintendo Event.

Shaymin: Use Oak's Letter on a white rock on Route 224, opening to Shaymin's Paradise where he awaits you at level 30.

Arceus: Play the Azure Flute at the top of Mt. Coronet, standing on the strange symbol in front of the pillars where you encountered Dialga/Palkia. Awaits you at a menacing level 80, complete with theme music.

Darkrai: Bring the Member's Card to the Harbor Inn in Canalave City, and spend the night. Your character will be put in a nightmare on Newmoon Island to find a level 40 Darkrai.

Of course, there's always the simple option: trade.

When you trade, be aware that many of them, if not all, were created with a program like PokeSav. For the most part, you're going to have to cheat for the pokemon, or browse certain trading forums for a dude with AR/PokeSav to have a big giveaway for the legendaries.

Patience, my friend.

Answer 2

All 3 are only attainable from events, promotions run my Nintendo. There has been none to date.
In earilers versions of the Japanese game, a glitch was found which let you catch them, this was fixed soon after.
The only way really to get it currently is to buy an Action Replay, and cheat to get them.

Answer 3

Through Nintendo Events. And so far, no nintendo events announced.