Just wondering if it is possible to install Gnome3 under Kubuntu 11.04?
When you say Gnome 3, you probably mean Gnome Shell, right? Because those are different things. Gnome 3 includes Evolution, Nautilus, Epiphany, etc. Gnome Shell is just that.
But yes, you can install anything in any Ubuntu; you can install a desktop environment on Ubuntu Server, you can install all services on all desktop variants, and you can install all desktop environments in other desktop environments. Or you can just install parts of them. I sometimes use Xfce4 Panel in my Unity session, to add a bottom panel, for instance.
Just install http://apt.ubuntu.com/p/gnome-shell and then you can choose it when you log in.