You can't find the Kami trio in-game in B2W2; Thundurus is only available in White, Tornadus only in Black and Landorus if you have both Thundurus and Tornadus in one version.
The only way you obtain the three of them in B2W2 is by either:
-trading them from BW
-trading for them from another player
-using Dream Radar, which requires a 3DS