There are at least two examples of polysyndeton in chapter three.
Identify one of them and explain how it contributes to the impact of the chapter.
Polysyndeton is the repetition of several conjunctions used in proximity. It usually has the effect of speeding up a sentence, implying a sense of urgency or activity.
In chapter 3, one example occurs in Nick's description of Gatsby's parties:
The bar is in full swing and floating rounds of cocktails permeate the garden outside until the air is alive with chatter and laughter and casual innuendo and introductions forgotten on the spot and enthusiastic meetings between women who never knew each other's names.
This quote serves to emphasize the shallowness of Gatsby's guests and that social scene in general. There is an atmosphere of deception and illusion evident in the appearances and conversations of the party-goers, as reflected in the immediately forgotten introductions, and the excited meetings of people who don't actually know each other. The repetition of "and" hurries the sentence, adding to the frenzied feeling of the party description. It also implies that so much is going on at these events, there's no way to ever really slow down. The sentence almost swirls around the reader, much as the party atmosphere would swirl around the attendees.
It's importance though, lies in the surface nature of the interactions. Even though there's always something going on, no conversation, interaction, meeting, etc., ever has true meaning at these parties. Thus, everything is described as "casual": people "chatter" instead of conversing in a meaningful manner, no one makes any real attempt to remember those they meet. It reflects the shallowness of Gatsby himself, and of the West Egg society in general.