Fedora
Just install or try it live to use GNOME 3.
openSUSE
GNOME 3.2 is the default desktop environment of openSUSE 12.1.
Mageia
GNOME 3 will be part of Mageia 2. Install ‘task-gnome’ after installing Mageia.
Arch Linux
Arch Linux has GNOME 3 in the extra repository.
Ubuntu
From Oneiric (11.10) onwards, GNOME is just a click away.
Debian
GNOME 3 is available from the wheezy/testing repository.