Part-3 of our PixInsight tutorial series moves on to more advanced topics. Among those covered will be: HDR composition, building mosaics, multiscale processing via wavelets, Narrowband image processing, and further refinement of techniques involving Masks, PixelMath, etc. Also featured is a recap of earlier concepts detailed in Part-1 and 2, providing the much needed Context and Chronology - the what to do, and when to do it.