Right now, a 3 finger drag (no tap, no click) can be used as a left click and drag on a trackpad if set appropriately in the Accessibility settings. Otherwise it just acts like a two finger scroll.
I don't suppose it's possible to make a 3 finger drag act like a middle click and drag instead? I know that you can do a middle click and drag with a three finger press and release (but not completely removing your finger from the pad), but I don't usually let my other fingers up before dragging around so I get some scrolling- actually zoom in my CAD program. Why have the zoom mapped to both two and three finger drags? Make the three finger drag do something different.
Edit to add: Could the JSON from this middle click gesture be changed to work even before the pad is clicked? Emulate a real middle click (including dragging) with the trackpad. E.g. for CAD programs.