I installed Ubuntu 13.04 the one from their website, then I installed Gnome 3 to try it but I don't have a login screen it just goes to unity. I did this before and I had the option on the login screen to choose unity or gnome or gnome classic but I don't anymore. If anyone could help that would be awesome!
Go to the users section of your system settings and edit your account. Uncheck "automatic login" (may be labeled different, I'm using the german version). At next boot, the lightdm-greeter should appear and let you choose your favourite desktop environment.