From the very beginning of Act 3, it is made clear that things are amiss. Benvolio warns the weather is conducive to a fight and suggests they should leave the area. This makes the scene dramatic.
BEN: I pray thee, good Mercutio, let's retire. / The day is hot, the Capulets abroad. / And if we meet, we shall not scape a brawl, For now, these hot days, is the mad blood stirring. (III.i.1-4)
However, they do not leave. Mercutio scoffs at Benvolio's suggestion they should depart. This, of course, costs Mercutio his life. Mercutio's death is the first literal death in the play. There have been metaphoric deaths, like the death of Romeo's love for Rosaline, but no literal deaths until the third act.
MER: A plague o’ both your houses! I am sped.(III.i.90)
Mercutio famously calls a plague on both the Montagues and Capulets, which comes true. This is the most dramatic point in Act 3, Scene 1. Mercutio's call for a plague foreshadows the rest of the play, as well as hints back to the prologue that suggested the deaths that would follow. This line is crucial and is intertwined with many dramatic devices, like foreshadowing, catharsis and dramatic irony.