Python 3 installation
Yes, you should set up virtual environments for python 2.7.13 and python 3.6. when you want to run python2, just activate the python 2 environments. Same for python3.
If you installed python using Anaconda, there are instructions in the Anaconda documentation for creating virtual environments.
If you did not use Anaconda, you can find instructions for creating virtual environments by googling on 'virtual environment python' and there are many hits.
I would advise against deleting any version of python. Macs come with a version of python installed that it uses and should not be removed.
I installed python3 to my my late 2014 iMac through brew.sh and my terminal is still defaulting to an older version of python unless I specifically prompt it with the command python3.  Is there any way I can fix this so that the terminal just automatically defaults to python3, without me having to type in python3 every single time?  I really need to run django, and the fact that my system is still defaulting to python 2.7 is really getting in the way.  Is there perhaps any way I can uninstall the older version of python?