How does Shakespeare prepare the audience for the death of Tybalt and Mercutio up until Act 3, Scene 1 of Romeo and Juliet?
We do find a few instances of foreshadowing that can be related to both Mercutio's and Tybalt's deaths.
First, the upcoming battle between Tybalt and Romeo is foreshadowed when Tybalt notices Romeo at the Capulet ball, feels insulted, and makes a vow to avenge himself, saying,
I will withdraw; but this intrusion shall,
Now seeming sweet, convert to bitt'rest gall. (I.v.96-97)
Since Tybalt was ready to slay Romeo at the party but was stopped by his uncle, we can assume that by "bitt'rest gall," Tybalt is saying he will kill Romeo.
Upcoming deaths are also foreshadowed by Romeo, although he mainly prophesies his own death. When Romeo's friends encourage him to crash the Capulet's ball with them, he states that he had a dream prophesying upcoming death, as we see in the lines,
[F]or my mind misgives
Some consequence, yet hanging in the stars...
With this night's revels expire the term
Of a despised life, clos'd in my breast,
By some vile forfeit of untimely death. (I.iv.114-118)
Though Romeo seems to be prophesying his own death in this scene by speaking of the life enclosed in his own "breast," it could easily be applied to other deaths as well.
Finally, the battle between Mercutio and Tybalt that kills Mercutio can also be seen as being foreshadowed when we hear Mercutio speak of Tybalt. After the night of the ball, when Benvolio and Mercutio are still wondering where Romeo is, Benvolio announces that Tybalt has sent a letter to Romeo's father. It is Mercutio who speculates that Tybalt is challenging Romeo to a dual when he states, "A challenge, on my life," which can be interpreted to mean, "I swear it is a challenge" (II.iv.8). However, since it is Mercutio who speculates about the challenge, even using the words "on my life" to mean "I swear," it can be said that this line is being used to foreshadow Mercutio's own challenge to Tybalt and his own death. In this scene, Mercutio also spends a great deal of time describing Tybalt's fighting skills, which can also be seen as foreshadowing their upcoming fight, as well as Mercutio's loss of life.