Comparing these films?

i have to do a comparison of the films "the great gatsby" (upon studying the novel), "The sting" and "a river runs through it". i'm looking for common themes and motifs that run through these 3 films. can anyone help me think of some? i have "clocks/page of time" and "coming of age"..but i don't know how to link those through all 3 films...would "american dream" work in all of them? help, please!! =]