Answer:
Fleance is first seen in Act II of the play. In his first scene, Banquo and Fleance arrive at Macbeth's castle late in the night. Banquo hands Fleance his sword before going to bed. By giving Fleance his sword, this signals to us that Fleance is serving as his father's squire, or young nobleman acting as an attendant.
Although we do not see Fleance again until his father is murdered, Macbeth does mention him to Banquo. Macbeth is planning the murders of Banquo and Fleance and asks his friend, 'Goes Fleance with you?' Macbeth wants to be sure that Fleance will be present when he sends his men to murder them.
In Act III, when Macbeth's men arrive to murder Banquo and Fleance, they first attack Banquo. Banquo is immediately worried about his son saying, 'O, treachery! Fly, good Fleance, fly, fly fly.' Listening to his father, Fleance flees and successfully escapes the murder plot.
After Fleance hides, we do not see him again in the play. Macbeth does express his disappointment that Fleance was not dead describing him as 'the worm that's fled.' Macbeth is relieved that he has killed Banquo but is worried that Fleance is still alive, knowing that the witches' prediction may still come true.
Finally, Fleance is mentioned once more in the play by Lennox. Following Banquo's murder, Lennox is relieved the Fleance was able to escape, saying 'For Fleance fled: men must not walk too late.' Lennox knows that Fleance would have been killed as well if he had stayed or was found.