3.12 is still in development, and 3.14 will is still very much in development as well. For instance, currently for Debian it 83/77% ready.
Gnome 3.14 is available for 14.10 (utopic) the Gnome Staging PPA (only Gnome 3.12 is available for 14.04), which is updated fairly regularly with staging releases of the. You can use the PPA with a few others as shown here. You could use the utopic PPA, but that would likely cause issues (on Trusty - installing Utopic when it is released may make it easier).
You can also get a image of Gnome from here - this however is based on a Fedora release (Fedora 21, which includes it anyway (and works great :), not Ubuntu. You could get 14.10 (which it also nearly ready), and use the gnome-staging PPA.
You can now get 3.14 in Utopic 14.10, as explained here:
GNOME 3.14 was released back in September and it includes some
interesting changes like multi-touch gestures for both the system and
applications, re-worked default theme, new animations as well as
various enhancements for the code GNOME applications. More information
HERE.
Unfortunately, Ubuntu GNOME 14.10 (Utopic Unicorn) ships with GNOME
3.12 for the most part (there are even some GNOME 3.10 packages, like Gedit or Nautilus) but, as usual, you can install the latest GNOME
(3.14) by using a PPA.
Unlike previous versions, installing GNOME 3.14 from the GNOME 3
Staging PPA in Ubuntu 14.10 doesn't break Unity however, there are
things that don't work as they should, like:
very large icons for some apps in Unity Dash and other places - screenshot;
the default Ubuntu theme (Ambiance) doesn't work properly with some applications (screenshot) and Adwaita doesn't support Unity's CSS
window decorations so you'll have to use a theme that supports GTK
3.14 and Unity, like Zukitwo, but even then you may encounter various issues like up/down arrows for indicators (screenshot);
Nautilus, Totem and Gedit use client-side decorations and the default GNOME menu, so they will look out of place in Unity.
And that's just on a first look so there might be many other issues. That's why I don't recommend using this PPA if you're using Unity!
As usual, the latest GNOME is available in the GNOME 3 Staging PPA and it's not considered ready for general use (you will encounter
bugs!), so make sure you read the PPA description before using it.
Follow the instructions below on your own risk and only if you know
how to fix your system in case something goes wrong!
There are two more things I should mention, so you'll know what to
expect:
on my laptop running Ubuntu 14.10, applications that use client-side decorations have a large shadow at the bottom
(screenshot).
This didn't occur in the two virtual machines I've tested the GNOME
3.14 installation under Ubuntu 14.10, but you may encounter this bug, and I didn't find out what's causing it;
CSD applications have minimize and maximize buttons for some reason, even though GNOME doesn't use this by default - you can
disable them via GNOME Tweak Tool > Windows > Titlebar Buttons.
Install GNOME 3.14 in Ubuntu GNOME 14.10
Warning: Please read the output before entering 'Y' for the "dist-upgrade" command below to make sure important packages won't be
removed and if the "dist-upgrade" command tries to remove important
pages, abort the installation and remove the PPA! Also, it's a good
idea to save the list of packages upgraded by using the instructions
below, in case you want to revert the changes and ppa-purge fails.
Currently, only the GNOME 3 Staging PPA is required to upgrade to
GNOME 3.14 in Ubuntu 14.10 however, some packages might be moved to
the GNOME 3 PPA later on and that might break things if you didn't
enable this PPA on your system, that's why by following the
instructions below, you'll add both the GNOME 3 PPA and the GNOME 3
Staging PPA.
That said, let's proceed
Install GNOME 3.14
To add the GNOME 3 and GNOME 3 Staging PPAs and upgrade to GNOME 3.14 in Ubuntu GNOME 14.10, use the following commands:
During the GNOME 3.14 upgrade (or purge) process, you may encounter an issue similar to this:
(gtk-update-icon-cache-3.0:29077): GdkPixbuf-WARNING **: Cannot open pixbuf loader module file '/usr/lib/x86_64-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders.cache': No such file or directory
If that happens, firstly install libgdk-pixbuf2.0-dev:
sudo apt-get install libgdk-pixbuf2.0-dev
And then fix this issue by running the following commands:
Reboot (a simple logout / login may not be enough so to avoid any issues, reboot your system).
(Optional) Install GNOME 3 applications not available by default in Ubuntu GNOME 14.10.
You may also want to install the GNOME 3 apps which are not available by default in Ubuntu GNOME 14.10 (Polari, Bijiben, Clocks,
Sound Recorder, etc.) - install them using the command below:
If for whatever reason you want to revert the changes made by adding
the GNOME 3 and GNOME 3 Staging PPAs, you can purge them (purging a
PPA downgrades all the packages from that PPA to the version available
in the official Ubuntu repositories and disables the PPA) using
ppa-purge:
WARNING: This will likely fail due to dependices, and if not might break stuff. You will probably want to backup any important data first in case. Using 14.10 (or Fedora) would be simpler.
3.12 is still in development, and 3.14 will is still very much in development as well. For instance, currently for Debian it 83/77% ready.
Gnome 3.14 is available for 14.10 (utopic) the Gnome Staging PPA (only Gnome 3.12 is available for 14.04), which is updated fairly regularly with staging releases of the. You can use the PPA with a few others as shown here. You could use the utopic PPA, but that would likely cause issues (on Trusty - installing Utopic when it is released may make it easier).
You can also get a image of Gnome from here - this however is based on a Fedora release (Fedora 21, which includes it anyway (and works great :), not Ubuntu. You could get 14.10 (which it also nearly ready), and use the gnome-staging PPA.
Also:
Edit
You can now get 3.14 in Utopic 14.10, as explained here:
Edit:
If you really want have a go at adding 3.14 to 14.04, you could manually add/edit the PPAs for 14.10 to your sources list:
WARNING: This will likely fail due to dependices, and if not might break stuff. You will probably want to backup any important data first in case. Using 14.10 (or Fedora) would be simpler.