参考http://askubuntu.com/questions/212132/i-cant-add-ppa-repository-behind-the-proxy

sudo add-apt-repository ppa:gnome3-team/gnome3
Cannot add PPA: 'ppa:gnome3-team/gnome3'.

该问题是由于是使用代理上网导致的

export http_proxy=http://username:password@host:port/
export https_proxy=https://username:password@host:port/

并且在 /etc/sudoers 末尾追加 Defaults env_keep="https_proxy"

之后再运行

sudo -E add-apt-repository ppa:gnome3-team/gnome3