It is incredible to Nick that Gatsby should be the host of such a riotous and raucous affair as the party he attends in Chapter Three and yet be so invisible. The first few pages of this chapter are full of speculation about Gatsby and who he is and what he might have done in the past, and it is only later on that Nick meets Gatsby without knowing it, until he is told the man he is speaking to is his host. Gatsby diffidently apologises, saying he is "not a very good host," yet this signals the way that he is very separate from the guests he invites and who fill his house with drunken revelries. This is perhaps best symbolised by the final image Fitzgerald provides of Gatsby in Chapter 3, which is when Gatsby bids farewell to his guests:
A sudden emptiness seemed to flow now from the windows and the great doors, endowing with complete isolation the figure of the host, who stood on the porch, his hand up in a formal gesture of farewell.
Note how this quote endows Gatsby with the "emptiness" and "complete isolation" that describes his mansion after the guests leave. This of course emphasises the fact that Gatsby is very different from the people who come to his parties, reinforced by his limited interactions with them during the actual party described in this chapter.
In this chapter, Gatsby acts in a generous way towards his guests. Nick describes, for example, how people do not wait for an invitation to one of Gatsby's parties. They just turn up and Gatsby is very accepting of this behavior. He does, however, formally invite Nick and sends a chauffeur to pick him up. Moreover, Gatsby also provides entertainment and copious amounts of food and alcohol for his guests to ensure that they have the best possible time.
Gatsby, however, is notable for his absence. He rarely interacts with his guests, and Nick has to wait a long time before he actually gets to meet him. This creates a sense of mystery around Gatsby that is further reinforced by his sudden disappearance from the party when he receives a phone call from Chicago.
Rather ironically, Gatsby admits that he is "not a very good host" to Nick—but, in fact, he is characterized by his generosity and, when he is present, his warmth and good humor.