The nuvi 3790 and 3760 are the exact same hardware. But based on some setting in ROM (serial, model or similar) it will enable 3D and voice control on the 3790 and not on the 3760. The firmware (software) running on both are the same. Seems to me that the Garmin is not sure what kind of 3700 series it actually is. I've read about a guy with a 3760 that after a firmware upgrade got a 3790 instead.
Have you messed about with the files in the .System folder?
There is a GarminDevice.xml on the Garmin. Try renaming to GarminDevice.bak and run the latest webupdater (download from Garmin.com to be sure). Version 2.5.1 and up has "ability to update devices with invalid GarminDevice.xml".