Looks like I posted in haste.
It occurred to me that I should have tried installing the driver but WITHOUT plugging in the USB cable to the Galaxy 'phone.
I unplugged the 'phone, de-installed the Samsung_ANDROID driver, and ran the SAMSUNG_USB_Driver_for_Mobile_Phones.exe again. I was watching Device Manager while this was running, and instead of installing one Samsung_ANDROID driver it installed three separate drivers. They went by to fast to write down, but they installed with a success code, and now device driver shows “SAMSUNG Mobile USB Composite Device” under USB controllers and “Galaxy J3 Prime” under Portable Devices and “SAMSUNG Mobile USB modem” under Modems, instead of the android driver under “Sound, video and game controllers” which was coming up before and not starting.
I haven’t done any testing or programming yet, but this appears to have at least fixed the installation problem.
If you tried to install the driver with your Phone or other device attached, try unplugging it, delete the old driver, and run the driver installation program again.