pip3 should be in the
Scripts path of your Python installation:
to find out where your Python executable(s) is/are located. The result should look like this:
You can check if
pip3 works with this absolute path:
if yes, add
C:\path\to\python\Scripts to your environmental variable
There is another way to install the pip3: just reinstall 3.6.
For python3.5.3, pip3 is also installed when you install python. When you install it you may not select the add to path. Then you can find where the pip3 located and add it to path manually.