Now libinput is up to version 2.18.1-1, and KDE doesn’t support three-finger dragging.
When to support three-finger dragging?
Hi! There is an existing feature request in the KDE Bugtracking System related to this topic: 433923 – Two/three finger drag functionality integration
This looks like something that will just require someone with the time, ability and interest to jump in and figure out how to implement 