If you install Debian from the full LiveCD/DVD .iso - it will have Gnome 3 installed - along with whatever other default software-set Debian decides to ship.
After installing from the LiveCD/DVD - If you want to use a different desktop, you can use the package manager to install whichever desktop/s you like and then uninstall Gnome3. All of the major desktop environments are available in Debians repos, along with a lot of less known ones.
Otherwise, if you use the .iso for the minimal, net based installer, you have much more flexibility in what you initially install.
The .iso for the minimal, net based installer is much smaller than the LiveCD/DVD image and requires a working internet connection in order to install. It downloads and installs the latest versions of the software from Debians repos.
The minimal installer will ask things like: whether you want to install a desktop (you do have the option of installing Debian with no desktop).
If you decide you do want a desktop, it will allow you to select one or even several DEs/WMs.
It gives you a lot of other options too - whether to install a web-server, or mail server, development software/libraries, text editors etc.
So you can get pretty much all of your system set up exactly how you want it from the minimal, net based installer.
In the past, I've used the minimal, net based installer to install the Debian base-system with no desktop and then manually installed everything else on top using apt. It takes a bit of time and work, but you can get a relatively pared-down, bloat-free system - running only the things you want. Other times, to save time - I've selected a desktop and made most of my software selections from the net installer and then added any other missing bits via apt!
If you want to use something other than Gnome3 - the minimal net based installer might make more sense to use, because it saves you from installing the system and then adding your desktop of choice and removing Gnome.
