So would I be correct in thinking that I should just run the .stl file through Meshcam and cut and paste the resulting file into the GCode window of Mach3?
It requires some user interaction but yes that is correct.
Is it possible to just rename a *.stl file as *.tap or *.3dp. etc., and expect it to load into the Mach3 GCode window and run?
No, that would not work.
(Use Windows Notepad to view a Gcode file (Mach3 / Gcode folder) and you should then be able to tell the difference between a Gcode and .stl, etc. files).
Did I mention that I'm slow with this?
No worries there. We all started at the beginning and I am still learning as I go.
Tweakie.