Can I use IDLE to write python programs instead of the terminal on Mac? The Mac terminal uses Python 2, and I don't know how to change the Python version.
Macs come with Python2 and this should not be removed. Installing a new version is fine as long as you use virtual environments. You should use the Anaconda distribution of Python, then setup and use virtual environments.

There are several sites, like this one which will walk you through the process. It's very easy using Anaconda. Another site is here. Once you get virtual environments set up, activate the one for the version of Python you want to run. You can then use IDLE or Atom to write and run your scripts. 

Using virtual environments also allows you to change which Python version is used at the terminal.

If this is unclear just leave a comment and I will write a message explaining this better.