First, remember that Nick is consistently a judgmental narrator, though chapter three does reveal some aspects of his character that we as readers have not seen until this point in the novel. During chapter three, Nick encounters quite the motley crew; because he, for the first time, attends one of Gatsby's parties, he has the opportunity to judge many new people at once. At first, his overall response to the number of guests at Gatsby's parties is one of awe and intrigue ("[...] men and girls came and went like moths among the whisperings and the champagne and the stars"), then he begins to make more critical judgments as he meets additional characters. For example, when Nick and Jordan encounter "Owl Eyes" in the library, they listen to the drunk man in astonishment, then interrupt him as he begins, for a second time, to tell them about the books. Their abrupt escape from the library suggests Nick and Jordan share a mutual feeling of indifference when it comes to Owl Eyes.
Nick, as narrator, also conveys judgments and emotions that are unique to his character. At the beginning of chapter three, Nick describes the scene at Gatsby's as full of people who "[...] [conduct] themselves according to the rules of behavior associated with amusement parks." However, after he "[...] [takes] two finger-bowls of champagne [...], the scene [changes] before [his] eyes into something significant, elemental, and profound." Though Nick continues to be leery of most of the party's guests, one -- the host, Gatsby -- actually impresses him. Once he meets Gatsby, even before he realizes who Gatsby is, Nick finds his smile one of the friendliest he's ever known, and after their introduction, Nick continues to narrate, "It was one of those rare smiles with a quality of eternal reassurance in it, that you may come across four or five times in life." Nick's fascination with Jay Gatsby begins.
The champagne doesn't completely hinder Nick from sharing his more negative judgments, however. Once he, with a crowd from Gatsby's party, sees Owl Eyes and his drunk driver have managed to amputate the wheel from their car in a careless accident, it is apparent that Nick has some distaste for the whole situation. He would most likely compare these party-goers to Tom and Daisy, when he, at the end of the novel, analyzes the married couple as being careless.
After Gatsby's party, Nick becomes more romantic in every sense of the word. He begins to love New York, "the racy, adventurous feel of it, and the satisfaction that the constant flicker of men and women and machines gives to the restless eye." Also, by the end of chapter three, Nick begins to develop romantic feelings for Jordan Baker, though she is "incurably dishonest."
Overall, in chapter three, we as readers realize -- even more -- the extent of Nick's critical nature, but we realize even more that Gatsby suddenly has a profound influence on Nick's outlook on life. After the party, Nick seems to approach his new life differently and with the same sort of hope for the future that we, later in the novel, realize Gatsby has, too. Though Nick's attitude and tone change depending on the circumstances, he remains critical of others and confident in the alluring power of Gatsby.