I don't use GNOME 3, but I fired it up in VirtualBox and had a look through the menus. A quick search of the Web also turned up a blog post on switching languages in GNOME 3.6
. The answer to your question appears to be:
1. Applications > System Tools > System Settings > Keyboard > Typing
2. Click on 'Layout Settings'
3. Click on the '+' symbol to add the input sources (keyboards) you want (Polish and Russian)
4. Click on the Shortcuts tab, then select 'Typing' in the left pane.
5. Click on 'Switch to next source' and press Ctrl+Space to set the shortcut.
6. Exit System Settings, and the currently selected keyboard language should now be shown on the Panel, and you should be able to click on it (or use the Ctrl+Space keyboard shortcut to switch languages).
However, if you do not see an Input Source (keyboard language) icon appear on the Panel when you set up a different keyboard (input source) using the above method then that might be a bug in the SL implementation of GNOME 3, in which case I would suggest you file a bug report (click on the link 'Bugs' at the top right of this page to do that).