The only information known is that the only way to get them is through a new game called Pokemon AR Searcher http://serebii.net/arsearcher/
Tornadus is said to have more of a focus on the defensive and speed stats.
Thundurus is said to have more of a focus on Special Attack.
Landorus is said to have more of a focus on Attack.
^What these three things mean is not fully known. It is safe to assume stat rises in the mentioned stats, but it is not known if stat decreases will also follow this. It is also not known if the stats mentioned will be the only ones increased or if they will just be the ones to get a huge stat boost.
Considering how Landorus looks, it has been speculated that its Flying type will be removed. It is also speculated that the other forms will have a type change to go along with this.