3rd monitor on Ubuntu 20.04 with Radeon HD 6670/7670 disabled

Question

I've been running Linux Mint 17.3 with 3 1920x1080 monitors (HDMI, DVI, VGA) with no issues for 2 years. I installed Ubuntu 20.04 on an additional SSD yesterday, and the monitor on the DVI is black. It is detected, and Settings > Displays allows me to configure it, and I can move windows to and from it, but the monitor itself remains blank.

Reboot to Mint, and the monitor works again. Reboot to Ubuntu 20.04, and it's blank.

I upgraded 20.04 to the ppa:oibaf/graphics-drivers drivers, but the video to the other 2 monitors is scrambled any time the DVI monitor is enabled, yet the 3rd monitor remains blank - so it's disabled at the moment. The AMD proprietary drivers aren't available yet for 20.04, so I can't check those.

I'm low on options. Any suggestions would be greatly appreciated, as I'd like to stick with Ubuntu for this LTS cycle.

lspci | grep VGA shows

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Turks XT [Radeon HD 6670/7670]

xrandx shows

Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384
HDMI-0 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 510mm x 290mm
   1920x1080     60.00*+
   1600x1200     60.00  
   1680x1050     59.88  
   1280x1024     75.02    60.02  
   1440x900      74.98    59.90  
   1280x960      60.00  
   1280x800      59.91  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   832x624       74.55  
   800x600       72.19    75.00    60.32    56.25  
   640x480       75.00    72.81    66.67    59.94  
   720x400       70.08  
DVI-0 connected (normal left inverted right x axis y axis)
   1920x1080     59.93 +  60.00  
   1680x1050     59.88  
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1152x864      75.00  
   1024x768      75.03    60.00  
   832x624       74.55  
   800x600       75.00    60.32    56.25  
   640x480       75.00    59.94  
   720x400       70.08  
VGA-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 476mm x 268mm
   1920x1080     60.00*+
   1680x1050     59.95  
   1600x900      60.00  
   1280x1024     60.02  
   1440x900      59.89  
   1280x800      59.81  
   1280x720      60.00  
   1024x768      60.00  
   800x600       60.32  
   640x480       59.94  
   720x400       70.08  

Answer 1

For future reference: After trying a very large number of combinations, I discovered that using the default drivers (removing the AMD drivers per 3 Displays Ubuntu, ATI Radeon) and setting the LG monitor (on the DVI port) to 59.93 Hz with both Adjust for TV and Fractional Scaling enabled, and scaling set to 100%, results in a stable 3-monitor configuration.