I have just installed Python 3.5.1 on my Mac (running the latest version of OSX). My system came with Python 2.7 installed. When I type
Since Python 2 and 3 can happily coexist on the same system, you can easily switch between them by specifying in your commands when you want to use Python 3.
So for Idle, you need to type
idle3 in the terminal in order to use it with Python 3 and
idle for using it with Python 2.
Similarly, if you need to run a script or reach a python prompt from the terminal you should type
python3 when you want to use Python 3 and
python when you want to use Python 2.