Who enters in Act I, Scene 3?
There are two characters that make their first appearance in Act 1, Scene 3: Juliet's nurse and Juliet. We get a first glimpse of the nurse's frank and talkative character in this scene. In a lengthy speech, the nurse reveals that she had a daughter named Susan who passed away. Thus, in many ways, Juliet has replaced the loss of her daughter.
Juliet's obedience is revealed upon her entrance in Scene 3 when her mother desires to speak with her about marriage. Although the nurse and Lady Capulet believe that Paris, Juliet's suitor, would make a suitable husband for Juliet, Juliet reveals that marriage is not something she is dreaming about. Nevertheless, Juliet tells Lady Capulet that she will consider Paris, and that she will only pursue her interests with the consent of her mother. In addition, Juliet's tender age of thirteen is reiterated in this scene, which further reinforces her immaturity and lack of experience in matters pertaining to love.
So are you asking who appears in the play for the first time in this scene? If so, the correct answer for this is the nurse. She is Juliet's nurse, meaning that she has taken care of Juliet from the time Juliet was born. In some ways, she has been more of a mother to Juliet than Lady Capulet has been.
As an example of this, we find out, in this scene, that it was the nurse, and not Lady Capulet, who breast-fed Juliet when she was a baby.
The nurse will be a comic presence in the play but will also help Juliet get together with Romeo.