How to make Left Ctrl (while pressed) as key to choose 3rd level (Alt Gr key)

Question

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.

For example:

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)

Answer 1

There doesn't seem to be a way, at least not an easy one. These are the prepared options:

$ cat /usr/share/X11/xkb/rules/evdev.lst | grep lv3
  lv3                  Key to choose 3rd level
  lv3:switch           Right Ctrl
  lv3:menu_switch      Menu
  lv3:win_switch       Any Win key
  lv3:lwin_switch      Left Win
  lv3:rwin_switch      Right Win
  lv3:alt_switch       Any Alt key
  lv3:lalt_switch      Left Alt
  lv3:ralt_switch      Right Alt
  lv3:ralt_switch_multikey Right Alt, Shift+Right Alt key is Compose
  lv3:ralt_alt         Right Alt key never chooses 3rd level
  lv3:enter_switch     Enter on keypad
  lv3:caps_switch      Caps Lock
  lv3:bksl_switch      Backslash
  lv3:lsgt_switch      <Less/Greater>
  lv3:caps_switch_latch Caps Lock chooses 3rd level, acts as onetime lock when pressed together with another 3rd-level-chooser
  lv3:bksl_switch_latch Backslash chooses 3rd level, acts as onetime lock when pressed together with another 3rd-level-chooser
  lv3:lsgt_switch_latch <Less/Greater> chooses 3rd level, acts as onetime lock when pressed together with another 3rd-level-chooser

So Right Ctrl (and quite a few other) can be turned to a 3rd level key, but not Left Ctrl

Answer 2

1. Go to dash, then search Keyboard.
2. Select Shortcuts tab, navigate to Typing on left side panel.
3. Click Compose Key, change the value from Disabled to Left Ctrl.

4. Close Keyboard configuration, test your compose key then.

Open your favorite text editor, for example I'll use GEdit. Hit and release the Left Ctrl key, followed by word T and M. If the output show Trademark symbol (™), it means you have successfully enabled Compose Key.