How does the nurse differ in Act 3 to the first time we meet her in Romeo and Juliet?

At the point of the drama in Act III, the nurse has become
more involved  with what is happening between Romeo and Juliet.  She is much more of an
advocate for both children than the manner in which she is shown at the start of the
play, where we know her as more of Juliet's servant.  In Scene 3, for example, the Nurse
is stern with Romeo when he gives himself to his emotions and wants to harm himself. 
She forces him to stop and reexamine the situation and focus on what needs to be done. 
As opposed to someone who is blinded by the feud, the Nurse understands that both
children share strong feelings for one another and while what Romeo did was bad, there
is a relationship between both young people which guides her actions.  In the opening
scenes of the act, the Nurse is distraught with what happened to Tybalt.  Yet,
demonstrating the greater sense of advocacy that emerges in her character, she does not
let this blind her, but rather she understands the severity of the situation and becomes
a stronger resource for both children.  The nurse demonstrates herself to be a better
parent and caretaker than Juliet's own parents, later shown in the
act.