The Question and Answer section for The Great Gatsby is a great resource to ask questions, find answers, and discuss the novel.
I looked at chapter 3 again. The only think I can think of is Jordan's chat with Gatsby. She comes back and taunts Nick with some exciting news that she has.
At what point in the story are you referring to?
I think that Tom is implying that Gatsby gets his money through illegal means like bootlegging.