Works both for X11 and Wayland
In /usr/share/X11/xkb/symbols/<your_language> change the numerosign to numbersign
It will work even for Wayland, even though it's in the X11 folder.
The numerosign(№) is a symbol that is used in some languages, for example some slavic languages, and it is not used in others, for example English.
You may want to change your Shift+3 to type "#", if you don't usually use the "№".
It is especially useful if you use markdown a lot.
To change it, you need to edit the file /usr/share/X11/xkb/symbols/<your_language>.
For example, for Ukrainian, you would edit /usr/share/X11/xkb/symbols/ua.
Now change all the numerosign to numbersign.
Restart the session and now Shift+3 will type "#".