On Windows python behaves a bit differently compared to Linux and macOS. In lot's of situations you prefer the py command over python3 command. This is especially relevant when dealing with virtualenv on windows.

Check out this guide to see how things work out on windows https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/

As it has already been written in the comments, your PATH environment variable does not include your current virtualenv installation directory.