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 consider them. 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 who do exactly what they are supposed to do.