It’s no secret that Daenerys has a special relationship with the number three. This is most overt in the personal, prophetic and the miraculous aspects of her journey: she is third born, a child of three, comes to possess three dragon eggs/dragons, will light three fires, ride three mounts, suffer three treacheries, and the dragon has three heads. But the number three frequently reoccurs in the more mundane story as well: three hostile khals, three loyal Bloodriders, three Seekers, three unfaithful Pureborn, three (unhelpful) Qartheen trading associations, three Pentosi ships, three attempts to secure an army, three Slaver cities, three Storm Crow commanders, three pickled heads from Mantarys, three surviving Dornishmen, three warlocks in the service of Euron III, three hostile Volantene Triarchs, three Westerosi passengers aboard the Selaesori Qhoran, three thralls to blow the dragon horn for third-born Victarion, and three remaining hostages in the Yunkish camp (as Dany herself bemoans, so many threes). In the more important groups of three a third event or person breaks from the first two. The third Seeker Quaithe actually gives Daenerys helpful advice, the third attempt at gaining an army in Astapor is successful, the third Storm Crow commander turns his cloak, Dany decides to stay and rule the third slaver city, one of her three dragons bonds with her and flies off, and so on.
Now, this special relationship isn’t all that useful when it comes to interpreting the narrative and making predictions. For the most part this is a pattern that reveals itself as the story unfolds, to be enjoyed in hindsight. The number three might be a reoccurring, symbolically important feature of Daenerys’ storyline, but we have little idea how it will reoccur absent heavy foreshadowing, nor how important its manifestations will be to the overall narrative (these details are all locked up in Martin’s head). There is however one case where we actually do have some rather heavy foreshadowing whose narrative importance is fairly obvious once uncovered. This foreshadowing has to do with the fate of Qarth.
Given Dany’s special relationship with the number three, it seems fairly significant that Qarth has three great walls and three great gates that would have to be overcome in order for the city to be captured. It is equally significant that Dany has so far had two formal audiences in which she dressed up in Qartheen fashion, one with the Pureborn and one with Xaro (who in addition to representing the Thirteen was also serving as Qarth’s ambassador). She has also had two personal encounters with Xaro Xhoan Daxos.* A third audience with the Qartheen and a third encounter with Xaro would be the result of Daenerys burning down Qarth’s three gates and sacking the city. The whole city is therefore symbolically setup for her violent return.
*whose name makes him one big walking three, amusingly enough.