Is there a way to make Left Ctrl (while pressed) as key to choose 3rd level (Alt Gr key)? In Ubuntu/Gnome/Cinnamon, there isn't an option for it in the Keyboard Layouts Options: Cinnamon-keyboard-layout-options
Swapping Left Control and Window key, and then making the "new" Window key (while pressed) as a 3rd level key doesn't seem to work.
Edit: I edited the question to change Compose key to key to choose 3rd level (Alt Gr key) and add "while pressed" to it. Compose key and 3rd level key are different.
C → c (lower case — first level) ⇧ Shift+C → C (upper case — second level) AltGr+C → © (copyright sign — third level) AltGr+⇧ Shift+C → ¢ (cent sign — fourth level)