In the terminal run
sudo apt-get install idle3
(The right package name for IDLE with Python 3 is
idle3 I think you can check it Software Center.)
If you are not sure about the name of a package you can run
apt-cache search <string> which will look for packages which contains
<string> in their names or descriptions.
To use Python 3.3 in 12.04:
- You can try bootstrapping Raring/Saucy. See pbuilder HowTo. (Kind of complicated, but very good.)
- Or you can try installing Debian+Xfce testing or Xubuntu 13.04 in VirtualBox. (This depends on your system capabilities, that's why I recommend a lightweight DE.)
- You can even try building Python 3.3 from source and installing it in a folder of your preference, but don't know how much effort this would take.
Of course, the most 'trivial' alternative would be to upgrade from Precise to Raring. Specially good if you can backup your important data and do a clean install.