Hi Enrique,
I have not tried to focus a 135 on my IIIc - the longest focal length I have used on it is 9cm - but further to the good advice you have already been given, can I suggest you get yourself an OKARO.
This is the orange filter that you can see in the picture below.
My IIIc.jpg
Mounted over the rangefinder window it turns the "mobile" part of the double image orange. This helps you to differentiate when you achieve sharp focus, because the orange tinge disappears when you do.
They are relatively inexpensive, and turn up on eBay regularly.
As a footnote, I actually find it easier to focus my IID than my IIIc...!
Regards,
Bill