If you have any conflicting ideas to the theories put forth in this, please feel free to discuss it here or post them under an appropriate title on the wiki page.
Surprisingly little has been done about 3 gates itself so you seem to be going into new territory.
Upon reviewing the page as it stands - I accept the postulation that the three gates are the outer, inner, and upper gates respectively. This makes the most sense.
My dividing the curse into three distinct dichotomies it was intended to - and did - sow confusion among the defenders who would be encountering new and unexpected combatants during any subsequent defense (those who were cursed either retreating or standing in defense, but not familiar to *anyone* on their side, since not even their armor would fit anymore).
First curse: Age Regression
Laid upon the Outer [Euper] Gate, intended to completely decimate any defense there by rendering the defenders into infants.
Second curse: Transgender
Laid upon the Middle [Keeptowne] Gate, intended to sow the greatest degree of confusion among the refugees and defenders with everybody suddenly turning into insanely gender-confused nymphomaniacs.
Third curse: Animorph
Laid upon the Upper [Keep] gate, intended to set the defenders (who would be relatively out of immediate reach of Nasoj's assaulting force) on each other as predators, prey, and completely scared out of their animal minds. This was dumped on Kyia's doorstep and almost on the very center of the Metamor manna-node. This would explain why the animorph curse is the most often manifest; it's closest too and most intrinsically entangled with the magic of the Keep.
This was, of course, wildly successful. Unfortunately for Nasoj, Metamor's greatest defense-mages were assembled within the Great Hall and were able to marshal defensive magics against the results, but not the actual spells that had been cast. Their response was entirely reactionary and they could not protect themselves from the full brunt of them. They perished in the counter-magics, burned out by the energies coursing through them while they were being warped by all three curses at once.
Also, if the spells were laid one after the other, rather than simultaneously (or nearly so), I suspect that unless the AR curse was laid last, that there would have been no survivors from that gate. It doesn't exactly take a lot of effort to kill a baby, after all…
Now, if the Keep is hexagonal, that would allow for three gates.
As we all know, the death toll during the Battle of the Gates was considerable, but the brunt of it did not likely occur until after the curses. The solid defense of the outer wall, and guerrilla fighting within and around Euper being the reason that the curses were necessary. Nasoj could not break the defenders through force of arms or front-line magic.
Yes, the Age-regressed suffered the highest losses, regardless of the timing of the curses. Yet they were not totally overwhelmed because, even with the defenders removed from the walls, the attackers still had to contend with those walls. And, if they wanted to overcome Euper, would have been forced to go house to house killing infants. The time to deal with either allowed the counter-magic to be cast, though losses were still high because they were still child-sized.
Houses where un-cursed children may have all too readily taken up arms if they had not been removed from the area long before the siege began. Places like the warrens and other slums would have not likely withdrawn except deeper into their labyrinthine streets where any attack would have met with stiff resistance from every quarter. Petty mages would have been able to contend with spreading fires if not the initial point of ignition.
The gender-switched rallied more swiftly, almost the moment their overwhelming lusts were countered. They turned their attention to the sorely embattled walls and managed to beat back the early foot-holds before they could become full breaches. These were likely the most heroic of the day, of all those who were cursed, for they suffered the least confusion and were in the easiest position to shore up the failing outer walls. Non-combatant women suddenly having the strength and fury of men, as well the wrath of a woman defending home and children would have made them frightening berserkers.
The animorphed, furthest from the immediate conflict, would have suffered the greatest confusion and some losses amongst themselves before they began to rally after the counter-magic restored their sanity and intelligence. Not suffering the decimation of the age-regressed, nor the losses of the gender-switched who charged to the defense of the outer wall, their numbers were not as heavily reduced in the following days of battle to secure Keeptowne and liberate Euper.
This, at any rate, is my take on it. A hexagonal bailey wall, with not one but three points of access to the same location, would be folly in a conflict zone. In the south where such warefare is not as common one might expect complacency to lead to such a difficult to defend design, but not the one point of access the war-like north had to the rich south. Thus, an outer wall might have multiple gates, but an inner wall would have fewer or one, and the Keep would have a single stoutly defended gate.
Hareford had 4 gates in the outer wall but (centuries ago?) two had been sealed off, limiting access points for attackers to focus on.
Hareford is built onto and into the remains of a Seuilman Fortress. The walls and gates are the original Seuilman ones. When Prince Ovid claimed Metamor in CR 230 they also reclaimed Hareford. Two of the Gates (one is the north gate. I forget the other. I think the east one) were sealed shut as being too dangerous.