How are both, the 3 letters and 3 jewels in the "Twelfth Night", a theme in the play?
In Shakespeare's Twelfth Night, much of the humor and confusion is created by the exchange of various letters and jewels.
The most prominent letter is the one Maria writes to make Malvolio, who is constantly preventing her and her friends from enjoying themselves, think that Olivia is in love with him. The result of this letter is that Malvolio makes a fool of himself before Lady Olivia.
Ultimately, Malvolio is locked away because he is considered insane. Eventually, his tormentors allow him to write a letter to Olivia to ask for his release from the "hideous darkness" into which they have cast him. Once Olivia receives the letter, she orders Malvolio to be released.
A third letter is also mentioned in the play. In this letter, Andrew writes to challenge Cesario (Viola in disguise) to a duel. Toby Belch, however, realizes that Cesario will not be frightened by Andrew's letter and decides to deliver Andrew's challenge orally in order to create a frightening image of Andrew's fighting abilities.
As for the jewels, early in the play, Duke Orsino gives Cesario (Viola) a jewel to take to Olivia as a token of Orsino's love for Olivia.
To her in haste; give her this jewel; say,
My love can give no place, bide no denay.
Next, Malvolio imagines that after he marries Olivia, he will become rich and will be able to play idly with "some rich jewel" while he treats Sir Toby as an inferior.
Finally, Olivia gives Cesario (Viola) a jewel to wear that contains Olivia's picture. The jewel serves as an outward and visible sign of Olivia's love for Cesario.
As for how the letters and jewels are connected to the play's theme, that is a matter of speculation. Both the letters and the jewels are associated with the confusing and frustrating nature of love. In the case of Orsino, Andrew, and Malvolio, they are all in love with Olivia, but she is in love with Cesario (Viola) and she eventually marries Viola's twin brother Sebastian.
If we look at the relationship between Orsino, Olivia, and Viola as a love triangle, then the number of letters and jewels matches the number of the lovers. Orsino loves Olivia; Olivia loves Cesario/Viola; and Viola loves Orsino.
Ultimately, true love is not won by letters or by jewels, but by revelation of the truth. Just as the holiday of Twelfth Night celebrates the epiphany of God in the form of Jesus, in Shakespeare's play true love is accomplished once the truth becomes manifest.