I installed them both via apt-get install and they seem to be the same. If I start them they both open up a python shell
idle3 is a package that depends on idle-python3.3 and Python 3.3 (in Quantal, probably 3.2 in older Ubuntus) amongst other things. From the description (from apt-cache show idle3):
This package is a dependency package, which depends on Debian's default
Python 3 version (currently v3.3).
The package itself provides a /usr/bin/idle3 script which imports the library provided by the idle-python3.3 package. idle-python3.3 provides a version specific /usr/bin/idle-python3.3 script too.
