GNOME 3 does not provide the same ease of customisation as GNOME 2 but does support extensions which be installed from the GNOME Shell Extensions website at https://extensions.gnome.org/ (See note below about Firefox GNOME Shell Integration plugin).
GNOME Shell extensions are produced by third parties. They are not an official part of GNOME. IT Services cannot provide support for issues which arise from the use of GNOME Shell Extensions. If an extension causes an issue, disable it.
If the GNOME Extensions website warns you "We cannot detect a running copy of GNOME on this system" you need to allow Firefox to run the GNOME Shell Integration plugin. There will probably be a prompt near the top of the Firefox window prompting you about this. If not click the Hamburger Menu icon (looks like three horizontal lines) to the right of the Search field and click Add-ons. In the Plugins section set Gnome Shell Integration to 'Ask to Activate' then reload the GNOME Extensions website.
GNOME Shell Extensions are installed in to $LOCALHOME/.local/share/gnome-shell/extensions/