If you really want to build the whole thing for hacking purposes, yes, Jhbuild is probably what you want. But if you just want to give GNOME 3 a try, I suggest

If you really want to build the whole thing for hacking purposes, yes, Jhbuild is probably what you want. But if you just want to give GNOME 3 a try, I suggest you to simply run one of our live images here: http://www.gnome.org/getting-gnome/