I'm using Gnome 15.04 currently.
Not for 15.04. You need Wily, available since of today, and at least one PPA
sudo apt-add-repository ppa:gnome3-team/gnome3
To get more bleeding edge versions of the components, additionally
sudo apt-add-repository ppa:gnome3-team/gnome3-staging
Update the package database and upgrade
sudo apt-get update sudo apt-get dist-upgrade
And if you have problems, start the commands below to go back to the version of the official Ubuntu repositories
sudo ppa-purge ppa:gnome3-team/gnome3 sudo ppa-purge ppa:gnome3-team/gnome3-staging
If you want update to Gnome 3.18, you'll need upgrade to Ubuntu 15.10.
On Ubuntu 15.04 you just can update only to Gnome 3.16, the actual version bundled with Ubuntu 15.10.
Follows the step by step valid to update to both versions: