This passage is an interesting one.  It plays into the automobile motif that runs throughout the novel.  We see autos as symbols of the American Dream--status symbols.  In this passage, for instance, Gatsby, according to Nick, is driving a "splendid" car.  

But more often we see cars in accidents.  At the first party of Gatsby's that Nick attends, Owl Eyes is involved in an accident.  The driver was so drunk that he thought he could drive the car even when a wheel had come off.  Later Jordan explains that she can be a "careless" driver because others are careful.  This more negative connotation of cars plays into the idea of the hearse--a foreshadowing of Gatsby's car that later becomes known as the "Death Car," when Daisy's careless driving kills Mrytle.  So, the mention of the hearse foreshadows the end of the novel--both the accident and its fatal consequences.   

The mention of the "three modish negroes" in a car that rivals Gatsby's is more difficult to explain.  Here we see the accessibilty of the American Dream.  It is possible for all types of people to achieve the material aspects of the American Dream--the big cars, big houses, big parties.  But it is more difficult, if not impossible, to attain the dream as Gatsby defines it--a return to youth, to infinite possibilities, to the "star-shine" of Old Money that "imprisons and preserves," and leaves one safe "above the hot struggles of the poor."  In other words, this passage distinguishes Gatsby's dream from that of others.  Turning back the clock is much more difficult than acquiring a "splendid car."