Should Three Swords manage to not only protect but eliminate 90% of the combatants, their martial prowess would tower over all others. Once the kingdoms knew that they were Dylan's allies anyone on the fence would grant him their support.
Even if they did not like him, they would show support just so Hiro's battle harem do not attack them. With the support of so many kingdoms even if he did not have a lot of allies among the nobles the pressure Rhyne's side needed to deal with would be immense.
In the feudal ages, there were very few kingdoms that maintained armies. That's because any hand training would a blade would not be able to farmland.
Thus in proportion to strengthening a kingdom's army, they would lose possible produce. The battle harems solved this as they were an army that rarely needed salaries.
They fought not because of coin but devotion. Well, that was ideally the case. Battle harems for hire similar to mercenary guilds also existed as a minority.
