If the slits are close enough (as with the 2 slit experiment) you get superposition from all three slits. The pattern is more complicated but the principle is the same. Where the waves can reinforce you get a brighter image, where they cancel a dimmer image. It does not matter that you arrange to have a low photon density so that the set-up only has one photon in transit at any one time, it is the probability amplitudes that interfere. You can substitute "more accumulated hits" for the word "brighter".