Over time the internal mirroring system in the M3's rangefinder might get dusty, or mirrors might even deteriorate.
I had the top off my M3 (used specially bought custom fit tools!) and have found that when the mirrors are somewhat coudy or dusty, the rangefinder patch improves greatly when you wipe two mirrors that are in plain sight with a cotton swab, very gently. Also, clean the inside of all windows before reassembling.
With the right tools, this is a DIY job if you are very careful, since you do not want the mirrors to shift. IMHO, cleaning a sensor is more stressing and many of us consider this a DIY job as well.
But, for Pete's sake, avoid marring your camera and get the right tools! These might set you back the price of a CLA anyway on evilbay. Your choice.
Happy shooting!