Fred,
I don't recall a thread on using a third axis for the lathe and have not done one for an individual live tail stock as it's different than using live tooling
mounted on the carriage.
Someone else may post on the matter but until then here are few of my thoughts:
You can use the mill screen so you have 3 axes or use Phil's screen ( it provides an A axis, thus everything A would need to be changed to Y ).
I would recommend a custom lathe screen be done.
So any gcode done for the tail stock would be for the Y axis. ie; if you generated drilling code for Z you would need to change to Y.
FWIW,
RICH