I just got the three orbs from Ghetsis's Shadow Triad, so how do you get Giratina, Dialga, and Palkia in POKEMON White version?
You can't get Dialga, Palkia or Giranta in B/W in-game. The only way like said by Fanny Pack is to transfer. The reason for this is that you can't trade items over pokétransfer so the only way to get the Trio specific items is by the Shadow Triad.
You can't get them in the game. The only way you can is by trading or transferring from Gen IV games. I hope I helped