Installing GNOME 3.12?

(Or, perhaps, 3.14...?)

Sorry for the double-post!

I think the best way is to upgrade to Trisquel 7 (once it's ready, or if you are brave enough) and add a PPA (GNOME testing/staging PPA, etc.).
Using PPA is risky because it may contain proprietary sotware and unstable software. If you want to use it anyway, I strongly recommend you to install also the "ppa-purge" tool.

This is how I installed Gnome 3.12.2 on Trisquel 7 20140807 testing version. It have not been running for over one month. If this will work at Trisquel 6.0.1 I do not know. Try?

sudo add-apt-repository ppa:gnome3-team/gnome3-staging
sudo apt-get install gnome-session gnome-shell
sudo apt-get update && sudo apt-get dist-upgrade

Source of my test
https://trisquel.info/en/forum/cannot-install-gnome-shell-trisquel-7-i18n-64-bit#comment-55679

Maybe interesting read about different ppas of Gnome.
https://wiki.ubuntu.com/UbuntuGNOME/Developers

I searched in Synaptic and I found Epiphany 3.12.0 and Gnome Maps 3.12.2.1. It must be in the Gnome PPA I have added. I do not know if the PPA I have added includes proprietary/binary blobs.

In Trisquel 6?

You don't, I'm afraid (or at least I don't).

Install Trisquel 7 instead (it's already usable, or at least the packages are since I don't install from the ISO), then try Garsmith's suggestion:

sudo add-apt-repository ppa:gnome3-team/gnome3-staging
sudo apt-get install gnome-session gnome-shell
sudo apt-get update && sudo apt-get dist-upgrade

If you want the latest GNOME, your best bet is probably Parabola. Do keep in mind, though, that Parabola is a very bleeding-edge distro, and it's said that the bleeding-edge is soaked with the blood of its users, or something like that.