I wanna know because if you could not what would be the point of giving you there orbs in the game?
You can't. You have to trade on GTS or Poketranser from D/P/Pt. You could also cheat. The reason they give it to you is because you can't transfer items over from earlier, so incase you do get them, you have the three orbs ready.
Dialga
Palkia
Giratina
You can't catch palkia/dialga/giratina in pokemon BW. You must do one of the following to obtain them: