From what I gather, you need to collect enough Dream Orbs to purchase items that allow you to catch the legendaries.
Edit: Oooor maybe not. You do need to play the game enough to unlock the Extensions (I got one "randomly"), but I don't think you have to purchase them. Also, you can pay 5 play coins to refill all your available Dream Clouds 3 times per day, which is kinda neat.
Also, is it just me, or do the Therian formes remind you of previous generation's legendary trios? Tornadus clearly looks like the Legendary birds, and Landorus looks like the Legendary Beasts from Gen II. Thundurus looks sort of like the legendary Sprites from Gen IV?