what does it tell us of the three kings who protested to Gulliver that in their whole reigns they never once preferred any person of merit , unless by mistake?

We learn that those in charge gained control by very dubious means.....

Perjury, oppression, subornation, fraud, pandarism, and the like infirmities, were among the most excusable arts they had to mention; and for these I gave, as it was reasonable, great allowance. But when some confessed they owed their greatness and wealth to sodomy, or incest; others, to the prostituting of their own wives and daughters; others, to the betraying of their country or their prince; some, to poisoning; more to the perverting of justice, in order to destroy the innocent, I hope I may be pardoned, if these discoveries inclined me a little to abate of that profound veneration, which I am naturally apt to pay to persons of high rank, who ought to be treated with the utmost respect due to their sublime dignity, by us their inferiors.