Arch Linux is a "rolling release" distribution that always gives you the latest software versions. Gnome 3 is the current version of Gnome. When Gnome 4 comes out, your Arch system will "roll" to Gnome 4, and so on.
I'm curious if you've read the Arch Wiki page on the topic?
"Fallback mode is very similar to the GNOME 2.x layout (while using gnome-panel and metacity, instead of gnome-shell and Mutter)."