You either trade from another game or if you got the special Arceus from the 2009 event at toys r us you can choose one of the three with their special orb at lv. 1.
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No, but you will be able to catch Palkia, Dialga or Giratina.
giratina
In Pokemon Diamond, Pearl, and Platinum, you cannot 'recatch' Dialga, but you can have multiple attempts if you save before battling Dialga. In Pokemon HeartGold and SoulSilver, when you do the Arceus event, you automatically obtain your choice of Dialga, Palkia, or Giratina.
No; Dialga, Palkia, and Giratina cannot fly.
the strongest Pokemon is arceus.then shymin,dialga,palkia and giratina.