Serial Python and Arduino

[delfar]

Putty user manual: https://the.earth.li/~sgtatham/putty/0.63/htmldoc/
Another thing that I noticed you are trying to open com3.
Usually the first com port is number 1, you normally only use com3 if you actually have more
that two com ports (two is common, 3 is not unheard of).
Make sure that you actually have a com3.
One way to find out is to look at your device drivers.
And while doing do, if you have a com3,  test the driver (there is an option in the drivers menu)

Thank you for the manual, 

The Arduino board is listed under COM3 and I have another connected to COM7