Turning keyboard LED lights on after boot

edit

You can add that command to the X11 startup.

Either globally for all users by creating /etc/X11/xinit/xinitrc.d/99-xset.sh:

echo "xset led 3" > /etc/X11/xinit/xinitrc.d/99-xset.sh
chmod +x /etc/X11/xinit/xinitrc.d/99-xset.sh

Or on a per user basis, by creating the file ~/.xinputrc, might exist already.

echo "xset led 3" >> ~/.xinputrc
chmod +x ~/.xinputrc

There is a command setleds. Just look up man setleds on a terminal. You can put the commands in a autostart file as suggested by @thomaswood. I'd prefer writing a bash script and adding it to your local account, e.g. by using gnome-tweak-tool or putting it in ~/.config/autostart.