Toon Boom does the converting manually which is especially helpful in your case because having files from two versions of Animate Pro is more complicated to deal with than files from one version of course.
If you’re working collaboratively it is recommended to all use the same version because often project go back and forth between animtors. Generally speaking the newer version can always open projects done using an older version but the older version cannot always open projects created in a newer version of the same software.
You can export scenes to Harmony (or Animate for that matter) from Storyboard Pro but you can’t open the actual Storyboar Pro project in Harmony.
With the exported scenes you get the camera moves, duration of the scene and visual reference.
I see Harmony 12 does not open Animate Pro V3 files. When you say toonboom does the converting manually what do you mean? Is there a conversion tool somewhere?