There are actually 3 legendary trios in black and white.
Tao Trio: Based on the Taoism premise of Yin (Zekrom), Yang (Reshiram), and the absence of Yin and Yang, Wushi (Kyurem)
Reshiram: catchable during the Pokemon Black storyline
Zekrom: Catchable during the Pokemon White storyline
Kyurem: Catchable in either game at the Giant Chasm off Route 13 after the main storyline is beaten.
Kami Trio: Based on the gods of Japanese myth Fujin (Tornadus), Raijin (Thundurus) and Inari (Landorus).
Tornadus: Available only in Pokemon Black. After obtaining 8 gym badges, go to Route 7 and talk to the old woman in the house where she was making soup before. Tornadus will then start roaming Unova (in the Southern Routes in the Morning/Afternoon and the Northern Routes during the evening/night). If you enter a route where a big storm is raging, check the tall grass and you will have a chance of encountering Tornadus. He will attempt to run immediately.
Thundurus: Available only in Pokemon White the same way as Thundurus.
Landorus: Take both Thunderus and Tornadus to the Abundant Shrine and you will find Landorus.
Musketeer Trio: based on the 3 musketeers from the Alexandre Dumas novel. Their names were Athos (Cobalion), Porthos (Terrakion) and Aramas (Virizion), as well there is a 4th musketeer, D'artagnan, represented by the event Pokemon Keldeo (not yet available).
Cobalion: Go to the Mistralton Cave, east of Route 6, to find Cobalion(you must have Surf). The other 2 musketeers will not be in their locations until AFTER you face Cobalion.
Terrakion: From the Pokemon League, go south to Victory Road. Go in the top cave and go down the stairs on the top left. Follow this path to find Terrakion.
Virizion: Explore Pinwheel Forest after face Cobalion to find Virizion.
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you find cobalin in mistralation cave, virision in pinwheel forest, and terrakin in victory road
You cannot get the "shiny beast trio" anymore, but you can still trade them to your copy of Pokemon Black or White to get Zoroark.
no
They roam the Sinnoh region after you talk to Professor Oak in Eterna City after getting the National Pokedex.
you will be able to trade at first once you get the C-Gear and the trio badge If you want to pokeshift you have to finish the game first then go to route 15 on the game just before you reach white forest or black city