Based on the information provided, it appears that you encountered an issue after downloading get-pip.py and modifying the environment variable path to "C:\Python27\ArcGIS10.3\Lib\site-packages". However, you have not mentioned the specific outcome or error message you encountered when running it from the command prompt.
To better assist you, it would be helpful if you could provide the exact error message or describe the outcome you experienced. With that information, I can offer you a more accurate solution to resolve the issue you encountered while running get-pip.py.