1There is no python3.exe file, that is why it fails.
2
3Try:
4
5py
6
7instead.
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9py 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
10
11py -2 or py -3
1Did you add python to PATH when you downloaded it?
2If not, redownload it and make sure you tick the box that says 'add Python to PATH'.