I think it's saying that the 3-slit case acts like 3 2-slit cases clasically superimposed on eachother. Each individual photon only goes through 2 of the slits

I think it's saying that the 3-slit case acts like 3 2-slit cases clasically superimposed on eachother. Each individual photon only goes through 2 of the slits and generates that 2-slit interference pattern, rather than going through all 3 slits and generating a very different interference pattern.