I'm not 100% sure, but I believe on Windows at least KiCad always uses it's own Python installation, and only that version.
If you are running a script in KiCad or using "import pcbnew" you must use the KiCad python.
But, if you are running a script independently of KiCad, then you can install Python3 and run scripts in windows Command Prompt.
In fact, I have the choice of 3 Pythons, KiCad Python, my own Python 2 install, and a Python 3 install. I use a batch file to set the PATH accordingly.