where is python installed windows

>>> import os
>>> import sys
>>> os.path.dirname(sys.executable)
'C:\\Python25'

You cannot just pip install python3 because it is not a library.

If you want to install the latest python3:
https://www.python.org/ftp/python/3.8.2/python-3.8.2.exe 

1. Open start menu and type python
2. Right click on python and select open file location. It will a window in explorer just like below.
3. Again , right click on selected python and select open file location. A window will open and you are now in python’s installation directory.