Is there a software setting you can switch to go from 'mirror' to 'extend'?
OR do you try that and it doesn't respect the 'extend' mode?
How much graphic memory have you assigned?
Also worth updating the guest;
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
The guest is up to date.
Shared graphics memory is: 768mb
Have you installed VMware tools?
$ vmware-toolbox-cmd -v
The problem went away after completely shutting down the instance and starting it, not just "rebooting" it. I should take the instructions more literal next time. After that, the whole screen was a little messed up (resolution, broken screen elements, etc.) but after logging out and back into Ubuntu that was fixed too. Now all three screens are recognized and have the correct arrengements and resolutions