You should ignore that message, choose "other" from the drop down list on login, input your credentials, then the message, "xsession unsupported number of arguments..." and you're in fallback mode since gnome3 - shell couldn't started at the moment. From there, you could try to enable the open source (ati) driver which would work best with gnome 3, but since you're on a live system i can't guarantee that your actions would have an effect.
All these messages, could be ignored and by-passed after installation. I know, i have tried my self.
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