This letter is addressed Dionysius the Younger. Plato remarks that he avoids the usual fawning expressions people address to tyrants:
For my part I should not address such an exhortation even to a man, far less to a god. To God it would be enjoining something contrary to nature, since the divine has its seat far removed from pleasure and pain; and as for man, pleasure and pain more often do harm, by breeding stupidity, forgetfulness, folly, and insolence in his soul. (315c)
Plato says that he has heard that Dionysius has been saying that while Dionysius wanted to repopulate devastated Greek colonies in Sicily and turn Syracuse from a tyranny to a monarchy, Plato prevented it. Plato replies that he did minor things that he thought would do good, like work on the preambles of the laws, but even those have often been revised by Dionysius. He recounts the story of his involvement in the affairs of Syracuse after Dionysius took over the city, noting Dionysius's repeated refusals to take his advice and recommendation seriously and his constant attacks on Dion, Plato's friend.
This letter is addressed to Hermias, Erastus, and Coriscus. Hermias was the tyrant of Atarneus, and may have been a student at the Academy, at least for a while; Erastus and Coriscus were certainly students at the Academy. (Aristotle repeatedly uses Coriscus as an example throughout his works.) Plato recommends that they support and benefit each other as friends:
Hermias should know that his power for all purposes has its greatest support not in the number of his horses or other equipment of war, nor in the gold he adds to his treasury, but in steadfast friends of solid character. And to Erastus and Coriscus I say, "old as I am," that they need to supplement their knowledge of the Ideas--that noble doctrine--with the knowledge and capacity to protect themselves against wicked and unjust men. (322d-e)
He ends with an interesting, if somewhat cryptic, remark, when talking about a covenant between them to be allies and friends:
Adopt it as a just and binding law and covenant, taking a solemn oath--in gentlemanly earnest, but with the playfulness (paidia) that is the sister of solemnity--in the name of the divine [leader] of all things present and to come, and in the name of the lordly father of this governor and cause, whom we shall all some day clearly know, in so far as the blessed (anthropon eudaimonon) are able to know him, if we truly live a life of philosophy. (323d)
(The Morrow translation in the Complete Works has 'divine letter'; I take it that this is a misprint for 'divine leader', since the Greek is hegemona, hegemon or leader.) We see here another manifestation of the idea of Plato having a special esoteric doctrine in addition to the exoteric doctrine of the dialogues.
This letter is addressed to Laodamas, who is a mathematician and student in the Academy. It mentions a Socrates, very possibly the Socrates the Younger who is a character in Plato's Statesman. Laodamas has asked for assistance in drawing up the laws of a new colony; Plato replies that he is too old to travel, and since Laodamas cannot visit Athens, and Socrates the Younger who might otherwise be a go-between is suffering from strangury, the most he can do is give some advice. The advice emphasizes the absolute importance of cultivating virtuous behavior in the citizens:
If they think that a constitution can ever be well established by the enactment of laws, of whatever sort they may be, without some authority in the city to look after the daily life of the citizens and to insure that both free men and slaves live in a temperate (sophron) and manly (andrike) fashion, they are thinking wrongly. This could be done, however, if you have at hand men worthy of exercising such authority; but if you lack an educator, then you have neither teachers nor learners, as I see it, and no course is left but to pray to the gods. (359a-b)
He notes, however, that many cities have only received this kind of good government after they were established, due to some circumstance or other in which "a man of nobility and character (kalos kai agathos) has appeared and exercised great power" (359b). The latter ends by saying that Laodamas should reflect on this, hoping for it, but should be careful not to assume that success is easily had.