Maybe it is too obvious, but I didn't find a single instruction on it. So here is how to actually do it:
1. Since the Ion3 package isn't included in the squeeze repositories, you will have to add the repositories of oldstable i.e. lenny. Just add the following two lines to your /etc/apt/sources.list (you need to be root to edit this file):
deb ftp://de.debian.org/debian/ lenny non-free
deb-src ftp://de.debian.org/debian/ lenny non-free
Note: ftp://de.debian.org/ is the german mirror, eventually you'll want to change it to a nearby mirror of your choice.
2. Update the cache:
#apt-get update
3. Install Ion3:
#apt-get install Ion3
