One of the most prominent themes in "Romeo and Juliet" is the destruction that comes from a long standing hatred, in this case, between two families.
Though the hatred between the Capulets and Montagues is made known at the outset of the play, it is not until Scene 3 that the seriousness of the feud is truly displayed. The fight in this scene starts in much the same way that the fight in scene 1 starts: with petty taunting and the exchange of insults. In fact, Tybalt is enraged by Romeo's newfound kindness. This result of his marriage to Juliet is misinterpreted by Tybalt as patronizing and ironic cruelty. Mercutio, not even a member of one of the families, with his quick wit and confrontational nature, encourages the fight that Romeo attempts to prevent.
This fight results in nothing but destruction. There are the immediate deaths of both Mercutio and Tybalt, which leads to Romeo running away, which leads to Juliet's faked suicide, Romeo's suicide, and Juliet's real suicide. There is also a skirmish in the meantime which kills Paris.
For a fight that started as something very petty (and almost silly) many serious consequences result. The emotions and irrationality shown by the characters in Act 3 are the culmination one of Shakespeare's main points of the entire play, which is to say that even the pettiest of conflicts, when properly fueled, can result in serious consequences.
I think fate/destiny is an appropriate theme for Act 3. In this Act, we see Mercutio wronged by the feud between the two houses. The prologue referred to the effects of the feud on the citizens of Verona as did the Prince in the first scene of the first act. We also see Romeo claim that he is fortune's fool and then he defies the stars after he kills Tybalt. These are each examples of the role of fate that the Elizabethans believed in and they convict us today to consider how a sequence of events can lead to certain consequences.
At this point in the story, Romeo gets banished and the "star-crossed" love is put on hold again not just because of the differences between the families, but because of Romeo's exile. It makes the audience wonder if this relationship was ever going to work out.