I have Linux Ubuntu 16.04 with several versions of python installed.
I managed to get what I wanted by adding this line to .bashrc:
which give the location of (1). However, do not delete or commented out the path to the anaconda. Hence, I can access (2) by using:
source activate root
and can easily switch to (3) by:
source activate py27
Note: py27 is the name I gave to (3) environment when I installed it using conda.
Then, if I want to go back to (1), just use: