This link pointed me in the right direction in case anybody else has this trouble:https://blog.reallusion.com/2016/07/25/speed-sculpting-a-character-with-zbrush-character-creator/
Basically Goz your model to ZBrush, paint your model, then PolyGroups, Auto Groups with UV. You can now shift-click to select parts of the mesh, just make sure you get all of the ones that correspond to the file you want to export.
You need to export each map individually, so if you are exporting the nails then select all of them (hands and feet) then Texture Map, New from Polypaint,Clone Txtr and export it as Nails.png or similar, you can then import that into CC3.
The biggest trouble I had seems to be a bug in ZBrush - if you click New from Polypaint and it clears the paint from your selected polygroups, undo the last step and then unselect the polygroups and reselect them, and then when you do New from Polypaint it correctly creates the texture map.
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