There is no python3.exe file, that is why it fails.
Try:
py
instead.
py is just a launcher for python.exe. If you have more than one python versions installed on your machine (2.x, 3.x) you can specify what version of python to launch by
py -2 or
py -3
You can also try this:
Go to the path where Python is installed in your system. For me it was something like C:Users\Local SettingsApplication DataProgramsPythonPython37
In this folder, you’ll find a python executable. Just create a duplicate and rename it to python3. Works every time.