from my website w ww .por tret teur .nl (remove the spaces):
Modern times: repair and buying checklist
The majority of the M3's is in need of service after 50 years. Even if you manage to buy a camera that is working fine at this moment, you should be aware of this. Compentent repairmen in the Netherlands charge approx. EUR 200 for a CLA (Clean, Lubricate, Adjust). Any additional work and parts are extra. There's also a number of competent repairers in the US, and most certainly in other countries as well.
Expensive repairs are those to the rangefinder system, which is the most complicated in the M3. When buying a 'new' M3, checking the rangefinder system is easy: look through the rangefinder window, both from the front and from the rear. You should see an evenly colored image. Lighter or black areas indicate one of the mirrors in the rangefinder system is in the process of de-silvering. Point the camera with the rear to a bright light source and look through the rangefinder-patch window (the small window at the front left hand side). Again, this should be clear from any 'debris' in the image, the image conveyed from the rangefinder window to the viewfinder is passed on by small glass prisms which are painted black on the ouside, 'debris' indicates that the paint on the prisms is flaking, causing the rangefinder patch to flare or lose brightness.
Other checks to perform when buying an M3:
* Check if the rangefinder image lines up correctly. Mount a lens and set distance to infinity, point the camera to a distant object and check wether the rangefinder lines up correct. If off horizontally or vertically, not to worry, this can be adjusted with the CLA. Perform the same check for a closest focus image.
* Check if film transport is even by cocking and releasing the shutter a few times.
* Check all shutter speeds. Start with the slow ones, cock the shutter a few times and release. One second up till 1/15th second should be clearly distinct, slow gears (buzzing sound) should release evenly.
* Remove the bottom plate and lens or body cap. Swing up the back door. Cock the shutter. Point the camera to a bright light source and look for any pin holes on the curtains. Release the shutter and check the second curtain as well!
* Cock the shutter again and inspect it from the front, you should see no creases on the shutter but a smooth cloth. Do not touch it with your fingers. Release the shutter and inspect the second curtain.
* Check if the take up spool is in the camera. I know, this sounds rather daft, but it's quite impossible (i.e. expensive!) to find one without a camera!
* Check whether the frame counter advances. I have owned a camera on which it didn't, and its a PITA since you always need to carry a spare film and run out of film when you're busy or not expecting it.
In general, the more yellow the frames and RF patch are, the more the Canada Balsam that was used in the rangefinder has aged. In time, this might lead to the RF failing: when the balsam-glued prisms break apart, the finder turns black. Sooner or later this can be expected in any M3, and a dent or bump may hasten the process.
Altogether I'd say all three of them are relatively expensive for what they offer. I've seen perfectly-usable and well-maintained M3's go on rangefinderforum.com for 800-900 USD.
Check your PM box.