What phrase in Act 3 is a simile?
Act Three of Shakespeare's Romeo and Juliet is riddled with similes. One of these similes occurs after the duel between Mercutio, Tybalt, and Romeo, in which Mercutio and Tybalt were both killed. The Prince arrives on the scene after Romeo flees, and Benvolio is left to attempt to verbally clean up the mess that has been made.
Benvolio explains to the Prince that Romeo had tried to end the fight between Tybalt and Mercutio but was unsuccessful in doing so. After Tybalt killed Mercutio, Romeo attacked Tybalt in a rage. Benvolio describes this duel as follows:
But by and by comes back to Romeo,
Who had but newly entertain'd revenge,
And to 't they go like lightening, for, ere I
Could draw to part them, was stout Tybalt slain.
I have placed the simile in bold typeface. Benvolio is figuratively comparing the beginning of Tybalt and Romeo's fighting to lightning in order to articulate how sudden it was. There was literally nothing Benvolio could have done to intervene given how quickly the situation escalated.
Another simile in Act Three, is when Mercutio, Benvolio, Page and the servants enter the public place in the street and Mercutio is fooling around with Benvolio. Mercutio tells him, "Thy head is as full of quarrels as an egg is full of meat; and yet thy head hath been beaten as addle as an egg for quarreling." Mercutio is teasing Benvolio about his eagerness to fight when Benvolio is not a fighter and very timid. I feel Mercutio is really talking about himself because he can be a hot-head if pushed far enough.
Source: The Literature and Language Book by McDougal Littell
Here's one, spoken by the Friar, in 3.3.130-134, in which he compares Romeo's intelligence to a careless solider whose gun powder explodes in his face, because he is either too inexperienced or negligent to treat it properly: