Thundurus: Pokemon White exclusive. I didn't catch it yet. I'm going to focus more on filling up my pokedex on White from old generations first; then I'll get to Unova. But Thundurus is exactly like Entei and Raikou in HG/SS as he roams Unova, but he is SO much easier to encounter.
Landorus: not sure.
Tornadus: Pokemon Black exclusive. Same as Thundurus.
Virizion: annoying since it has Giga Drain, and it heals itself after you weaken it. It took me a while to catch, but put a Chandelure up against it and it won't know what to do because either the moves don't affect Chandelure or they're not very effective. I accidentally made Virizion faint three times though. It's not the strongest of the trio.
Terrakion: again, uses the same moves over and over again, and much easier to weaken than Virizion.
Cobalion: easiest of the 3. Also uses the same moves all the time. Just watch out for Iron Head.