Thanks to a discussion on Facebook, I've solidified the V3 pinouts. For those of you keeping score at home, the probe is the old Z-Min pin (PB5) and the Z-min moves to PA2 on the Serial2 header. I'll have a code change for this/PR for Marlin soonish. I don't recommend using this on the Pro or V3 yet because the menu for adjusting the Z offset doesn't work - there's a whole new set of commands I need to implement (P:I, S:S, M:[0-998, 999], where S:S is save settings, M:0-998 sets a Z offset, and P:I initiates leveling).  Also, this will require the failsafe EXTUI support, which has to come first.