The three policemen don't really have any characteristics. Yet they play a major role in driving the plot of the narrator's story, so we should at least acknowledge the poor guys.
They seem to be conscientious – they waste no time in showing up at the house after suspicion is aroused. Furthermore, they don't leave after the narrator has given them the tour of the house, but seem to stick around to see what such pressure might induce. The three policemen are fairly unambiguous, flat characters...but at least they're good cops.