According the Prince in Act V Scene iii, in what ways are he, Capulet, and Montague punished? For what reasons are they punished?

PRINCE: This letter doth make good the friar's words,
Their course of love, the tidings of her death;
And here he writes that he did buy a poison
Of a poor 'pothecary, and therewithal(300)
Came to this vault to die, and lie with Juliet.
Where be these enemies? Capulet, Montage,
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love!
And I, for winking at you, discords too,(305)
Have lost a brace of kinsmen. All are punish'd.

Here the Prince explains to Capulet and Montague what has happened.  The "course of love" that he reads in Romeo's lover explains how Romeo Juliet met and married.  Since Capulet and Montague knew nothing of this, this is the first time they have heard of their children's love.

The "tidings of her death" summarizes the subterfuge that Juliet perpetrated to avoid marrying Paris, which was the ultimate cause of both Paris' death and Romeo's suicide.  Again, the Montagues and Capulets did not know that Juliet was alive until just a few minutes before this.  Then the Prince goes on tells of Romeo's description of his purchase of the poison and his intention to die with Juliet's body in the vault.  "Where be these enemies" is the Prince calling the feuding families of Montague and Capulet together.  He scolds them saying that heaven has taken their "joys" (children) away from them though the children's love for each other, as punishment for the feud.  The Prince also acknowledges his own guilt, in that he has ignored the feud and has therefore been punished by the death of members of his own family (including Mercutio.)  The families of Montague and Capulet have paid the dearest price with the deaths of their children , and therefore "all are punish'd".