How to set different scaling on multi monitor (3 monitors)

This works fine in my case, using 3k notebook and 1600px monitor with different scale:

1. open dconf editor
2. /org/gnome/mutter/experimental-features
3. Disable "Use default value"
4. write custom value: ['scale-monitor-framebuffer', 'x11-randr-fractional-scaling']
5. now you can use different scale for each screen in the display settings

dconf setting capture

scale 200% in screen 3000x2000px

scale 100% in secondary screen 1600x900px

test in: ubuntu 19.10 gnome 3.34.2 regards

If someone is looking for help with this in 2020 or later, probably the solution is to change your session to use wayland instead of xorg. It depends on your distro but you usually can choose what kind of session you want before logging in into your session. You should see some kind of cog icon and you should choose something like "GNOME on Wayland" or "GNOME" if there is some other option which says anything about xorg.

Please be aware that some apps might not change their scale when you move them from one screen to the other.