Chapter 3 is an amazing chapter. A lot happens in it. Probably the most profound thing that occurs is that Nick meets Jay Gatsby for the first time. Up until this point in the novel, Nick only knows Jay Gatsby from a distance. Through rumors and hearsay, Nick's impression of Gatsby is a bit mysterious. Chapter 3 doesn't diminish the mystery surrounding Jay Gatsby, but further deepens the mystery.
Nick is witnessing a HUGE amount of wealth and opulence at the party. Gatsby's house is amazing. The liquor is freely flowing, and the rumors about Gatsby are running rampant. Nobody quite seems to know much about the host of the party, but nobody has a problem having fun at his expense. Further deepening the mystery surrounding Gatsby is Jay Gatsby himself. He stands apart from most of the partygoers and isn't drinking. When Nick finally meets Gatsby, Nick has difficulty placing Gatsby's accent and notices Jay's odd habit of calling everybody "old sport."
The opulence of the party, Gatsby continuing to be a mystery, and Nick's intuition all alert the reader to the suspicion that Gatsby might not be exactly what he claims to be. There's one point in the chapter where a man is surprised that Gatsby's library books are all real. It isn't what other West Eggers would have. That comment and Jordan's comment to Nick about having learned something amazing about Gatsby make the reader wonder if the party and everything about it are just a facade for Gatsby.
Other minor events happens that are important to the story as well. Nick decides to begin dating Jordan despite knowing she is dishonest. Additionally, some of Nick's backstory and job are described. All of it together makes chapter 3 one of the biggest highlights of the novel.