If you're using Fedora 20, there's a repository maintained by Richard Hughes (the GNOME Power manager and PackageKit maintainer, among others) you can use to install the latest GNOME 3.12.
I tested the GNOME 3.12 Fedora 20 repository in VirtualBox and I didn't encounter any major issues (this is the repository I've used for upgrading to GNOME 3.12 in Fedora 20 to record the video in our GNOME 3.12 article) however, the repository maintainer notes that since the repository only had a small amount of testing, it's probably better to try it in VirtualBox rather than on a production system. So use it at your own risk!
If you want to try GNOME 3.12 in a virtual machine, you can also do this by using the GNOME 3.12 live CD.
That being said, let's proceed.
Install GNOME 3.12 in Fedora 20
1. Add the GNOME 3.12 repository in Fedora 20 by using the commands below:
su - echo -e "[rhughes-f20-gnome-3-12-i386]\nname=Copr repo for f20-gnome-3-12 owned by rhughes (i386)\nbaseurl=http://copr-be.cloud.fedoraproject.org/results/rhughes/f20-gnome-3-12/fedora-\$releasever-i386/\nskip_if_unavailable=True\ngpgcheck=0\ncost=900\nenabled=1" > /etc/yum.repos.d/rhughes-f20-gnome-3-12.repo
su - echo -e "[rhughes-f20-gnome-3-12-i386]\nname=Copr repo for f20-gnome-3-12 owned by rhughes (i386)\nbaseurl=http://copr-be.cloud.fedoraproject.org/results/rhughes/f20-gnome-3-12/fedora-\$releasever-i386/\nskip_if_unavailable=True\ngpgcheck=0\ncost=900\nenabled=1\n\n[rhughes-f20-gnome-3-12-x86_64]\nname=Copr repo for f20-gnome-3-12 owned by rhughes (x86_64)\nbaseurl=http://copr-be.cloud.fedoraproject.org/results/rhughes/f20-gnome-3-12/fedora-\$releasever-x86_64/\nskip_if_unavailable=True\ngpgcheck=0\ncost=800\nenabled=1" > /etc/yum.repos.d/rhughes-f20-gnome-3-12.repo
2. Upgrade to GNOME 3.12 in Fedora 20:
Once the update is completed, log out and log back in (though a system restart is probably better to avoid running into issues).