No, 2 filters fit. 

It fits exactly. The width is crucial, so with 100mm width it fits. 

No. This filter holder can only hold 2 filters. I believe the more prudent course would be to get a filter that is rated for more stops and can replace two filters that are rated for less stops. There are filter holders that do hold 3 filters but they are smaller than the K&F filter holder (which is 100 x 100 mm). And in my opinion, the quality of those 3 fitter holders that are located on Amazon is less than the K&F system. 

There does'nt appear to be a facility for holding a circular polariser on this system (i.e. the type you can rotate in the mount) 

If you plan to shoot during bright sunny days it is absolutely necessary. Its will save your highlights and let you open the lens to its widest aperture to get nice bokeh.