Python 3.5.1 is installed on Ubuntu 16.04 by default. You can verify that by running the command
python3 --version in a terminal.
Now, IDLE for Python 3 is in the package
idle3 and part of the default Python distribution but it's not installed on Ubuntu by default. You can do so with Software Center or your favourite package manager. On the command-line you can run:
sudo apt-get install idle3
If that gives you an error as you say in the comments, please edit your question with the "edit" link below it to include the error messages verbatim (and send me a message in the comments to notify me of the change), or ask a new question with the same information.