The 3mm thickness of the washers is designed to accommodate a flat-head screw, so it does not sit proud and consequently interfere with the required flat connection to the magnets surface.
The 3mm thickness of the washer will also ensure that the magnet can achieve its full magnetic force, compared to using a standard thin washer.
The 1.5mm thickness of the washers is designed to accommodate a flat-head screw, so it does not sit proud and interfere with the required flat connection to the magnets surface.