I have VoxATC running with my P3D V3. Used Prepar3D 2.5 before.
Just copied VA* from Drive:\Prepar3D v2\Gauges to Drive:\Prepar3D v3\Gauges and manually added gauges to Panel.cfg for each Aircraft I use.
Also copied VX* from Drive:\Prepar3D v2\SimObjects\Misc to Drive:\Prepar3D v3\SimObjects\Misc.
Finaly changed registry key AppPath and SetupPath for HKEY_CURRENT_USER\SOFTWARE\Lockheed Martin\Prepar3D v2 to the root folder of Prepar3D V3.
I also did run setup.exe of version 6.52 and let it repaire.
After all this run Indexer, Panel Setup, runway number updater and indexer.
Please do it at your own risk. I'm not using V2 anymore so I do not care if V2 is working properly.