I do not think Leica will work on an M3 anymore. I certainly may be wrong. I had Youxin work on my father's IIIf RDST and it came back recovered, factory fresh in 10 days door to door. It is in perfect shape.
Now my M2 is starting to peel, so after a bit I will send it to Youxin for recovering and while its there a CLA is always a good thing, although I think it has only been 8 years or so since KEH did it for me. They too did a nice CLA job, but I think Youxin is better by far.