What event in Act 3 caused the most problems for Romeo and Juliet? Why?

To me, the major event of Act III comes when Romeo kills Tybalt.  This is the turning point in the play and it will cause Romeo and Juliet to die.

After Romeo kills Tybalt, the Prince banishes him from the city.  Also, since Romeo has just killed Juliet's cousin, he and Juliet can't reveal that they are married.

Because of these things, Juliet has to pretend that she's going to go through with marrying Paris.  Then she has to agree to Friar Lawrence's plan to avoid marrying Paris.

Because Romeo is out of the city, Friar Lawrence can't tell him the plan for making it look like Juliet is dead.  Since Romeo think she's dead, he kills himself and she kills herself when she sees he's dead.

So Romeo killing Tybalt in Act III causes all these tragic events to occur.