Hello,
Sorry to hear you are having issues. The normal way should work. I did it few times on my test system. Just uninstall Fences 6, reboot (important), reinstall Fences 3, reboot again (important). Fences 3 should pick up the setting without issue. Except if you have tabs in Fences 6 setup. The tabbed Fences still appear but it will be on top of each other. You just need to move it away from each other.

By the way, interested to know, what on Fences 6 don't work for you?

Thank you,
Basj,
Stardock Community Assistant.