Hi there,
We are sorry that 3D isn't working in Photoshop 2020 for you.
Are the options in the 3D menu of Photoshop grayed out? If yes, please go to the Preferences for Photoshop, then Performance and let us know if the option to Use Graphics Processor is checked.
If the option is disabled, check if you have an update for your macOS available as the latest Graphic card drivers for Mac are installed with macOS updates.
Regards,
Nikunj