The last, and least, film in the so-called ''Uptown'' trilogy, this 1977 buddy comedy is preachier than its more rollicking predecessors, ''Uptown Saturday Night'' and ''Let's Do It Again''. It begins like ''The Sting'', but then veers into ''To Sir, with Love'' territory, as Dave (Bill Cosby), a safecracker, and Manny (Sidney Poitier), a con man, are blackmailed by a retired detective (James Earl Jones, who would achieve pop-culture immortality that year as the voice of Darth Vader in the original ''Star Wars'') to work at a community-improvement center and inspire the delinquent youth to respect themselves and find jobs. An answer to the violent and militant blaxploitation films of the period, these films, each directed by Poitier, were the ''Barbershop'' of their day. Poitier and Cosby are a seamless team; their ease, charm, and integrity carry the film's more plodding patches, as does Curtis Mayfield's authentic score. Fans of these icons will want to get a piece of this action. ''--Donald Liebenson''''''