Best assumption for how: (all post-game)
Beasts: roaming
Ho-Oh: Mt. Chimney Peak
Lugia: Seafloor Cavern
Hoenn legends: standard locations
Lake trio: just randomly scattered around a la BW2, or possibly roaming.
Creation trio: related to flying on the Mega Eon Duo
Regigigas: Sealed Chamber
Heatran: Magma Hideout cave in Mt. Chimney
Cresselia: Mossdeep City (Space Center? Psychic gym + space = Cresselia?)
Kami Trio/Forces of Nature: in that storm-cloud thing?
Swords of Justice: Virizion in Petalburg Woods, Terrakion in Granite Cave, Cobalion in Victory Road (?).
Tao trio: flying w/Mega Eon Duo
This game looks to be filled with enough Legendaries to outnumber BW2.
Of course, this is assuming that we don't get a metric ****ton of version exclusives like BW2's Eon Duo thing. Could easily get one of the members of each duo, one of the main two members of each trio and the third member (possibly similar to HGSS's Rayquaza), and either Cresselia or Heatran.