Any: [supported everywhere [tm]]
    Python 3.2.2 (via source)
    => solution for distros with python < 3.2

Arch: [supported]
    python 3.2.2-2

CentOS: [no python 3]
    Python 2.6.6

Debian: [supported in testing]
    Package python3
        squeeze (stable) (python): interactive high-level object-oriented language (default python3 version) 3.1.3-12: all
        wheezy (testing) (python): interactive high-level object-oriented language (default python3 version) 3.2.2~rc1-2: all
        sid (unstable) (python): interactive high-level object-oriented language (default python3 version) 3.2.2~rc1-2: all

Fedora: [supported since 2011-05-24]
    14: python3-3.1.2-14.fc14
    15: python3-3.2-1.fc15
    16: python3-3.2.1-1.fc16
    17: python3-3.2.2-8.fc17    

FreeBSD:
    python32 - 3.2.2

Gentoo:
    python 3.2.2
    
NetBSD:
    python 3.1.4

OpenBSD:
    2.7.1 on -current
    2.5 & 3.2 maintained in ports

OpenSuse [supported since 2011-11]
    11.4: python3-3.1.3-3.3.x86_64.rpm
    12.1: python3-3.2.1-5.1.3.x86_64.rpm

Redhat 6: [no python 3]
    See Centos 6

Slackware: [no python 3]
    python-2.6.6-i486-1.txz

Ubuntu: [>= natty, supported since 2011-04]
    Package python3
        lucid (python): An interactive high-level object-oriented language (default python3 version) 3.1.2-0ubuntu1: all
        maverick (python): interactive high-level object-oriented language (default python3 version) 3.1.3-3ubuntu5~really3.1.2: all
        natty (python): interactive high-level object-oriented language (default python3 version) 3.2-1ubuntu1: all
        oneiric (python): interactive high-level object-oriented language (default python3 version) 3.2.2-0ubuntu2: all
        precise (python): interactive high-level object-oriented language (default python3 version) 3.2.2-0ubuntu2: all