Photo Mechanic isn't actually used for any tethering on its own for any camera. A typical tethered workflow with Photo Mechanic looks something like this:
1: Configure tethering software (like the free Canon EOS Utility which works with Canon R3) to transfer images from the tethered camera to a certain folder
2: In Photo Mechanic, use Live Ingest (File > Live Ingest) and tell PM to monitor the folder you specified in your tethering software
Photo Mechanic's Live Ingest will then monitor that folder, and then do the typical PM Ingest tasks to the images as they come in, i.e. rename, apply a metadata template, route into other folders/subfolders, etc etc.
I hope that explains things a bit.
-Mick
I will attach a shot of my Live Ingest dialog. In this case I tethered my Fuji using Fuji's "X-Acquire" software and used Live Ingest to handle renaming the files and applying my standard metadata template.