To think I used to know this all like the back of my hand. The "standard" filter for the M3 is the 19mm Leica filer. You may have noticed it, but the front of the lens just surrounding the glass front element is threaded ---- it is here the 19mm threaded filter lens goes. You can also use one of the larger "Series" holders which slips over the entire front lens body, but this is a problem. Using the tiny 19mm filter allows you access to aperture changing and focus by turning the lens itself. Filters which cover the face of the lens can interfere with certain features. Those 19mm "mini" filters used to be made by Walz, and as I recall came in a set of four. They've become pretty scarce.
