What is it about the M3 viewfinder that is so complicated?
The M3 finder has several fancy prisms, including one that is a"laminated" large block that contains the permanent 50mm lines. Most of the problems you encounter with the M3 is cracked and fading lines in this prism and delaminating of the prisms, or "fungus' - which can look like a tropical growth! Most of the parts in the M3 finder are unique to this finder and as they are old now, parts are getting difficult to get, if at all possible. It is also a very costly finder to replace, around $700.
Leica can replace the 0,91 finder of the M3 with the 0.85, although I dont know how much it costs. Considering the age of the M2/M3 it is amazing how sturdy they are. There are a lot of these cameras out there with very good finders, so unless you have access to a "mintish" finder, stay away from the faded, blacked out ones, unless they are really cheap and calculate with at least $7-800 for fixing it.
The outlook for Leica's is better than the Nikon Rf's. It is virtually impossible to get parts for those finders. I have a mintish S2 black Dial with a defunct 2nd rangefinder mirror (it has the patch clearly defined, but the "swinging" mirror is dead). So far no luck in finding someone who can re-manufacture one of these.
At the moment it serves as a body for non-coupled lenses, 12/15 and a pre-war Tessar 28f8, but it would be nice to get it in working shape.