I'm trying to set up a convenient compose key to use for practicing Greek in duolingo. But, I don't want it to mess up my normal use of my laptop. I'm a developer by trade and use pretty much all of the normal single key options shown in the "Position of Compose key" settings in Linux regularly on my laptop, with the exception of the Menu key. However, my new laptop doesn't have a menu key. :-P
So, I wanted to try the "3rd level of Caps Lock" or any of the "3rd level" settings (see screenshot below for the settings I'm talking about).
But...having tried one, I'm not sure how to actually use it. I've tried searching, but all the info I've found about "3rd level keys" is when using a key as a modifier (e.g., holding Caps Lock and a simultaneously to get some modifier of a). That's not working though.
If I use one of these "3rd level of X" settings...how do I actually use it? :)
For reference, my distro is Linux Mint 19.3 (Tessa) using Cinnamon. Which is based on Ubuntu 18.04.