Some software wasn't working well in Catalina, and in particular one package needed a 3 button mouse for some funtionality. Actually on further investigation, it didn't - there should have been key, mouse or trackpad ways of achieving the desired result.
Oddly trying to set the Preferences for the trackpad on my laptop showed that the machine was expecting to connect a Bluetooth trackpad - crazy, as the MBP has one built in. Eventually I discovered that renaming the file "com.apple.BluetoothFileExchange.plist" as a .bak file in /Library/Preferences reenabled setting that device, and now the effect of a middle mouse button can be achieved with a two finger tap on the pad - which is what it should have been. The software now behaves as it is supposed to.
I don't know how this thing got changed, but if anyone finds that devices can't be set after installing Catalina, then do check the /Library/Preferences and if needed rename files, or even delete them, as suggested here. I think these do get reset on starting the machine up, so it should be a fairly safe process to try. In my case I didn't even need to restart the machine to recover the appropriate behaviours.
Oddly trying to set the Preferences for the trackpad on my laptop showed that the machine was expecting to connect a Bluetooth trackpad - crazy, as the MBP has one built in. Eventually I discovered that renaming the file "com.apple.BluetoothFileExchange.plist" as a .bak file in /Library/Preferences reenabled setting that device, and now the effect of a middle mouse button can be achieved with a two finger tap on the pad - which is what it should have been. The software now behaves as it is supposed to.
I don't know how this thing got changed, but if anyone finds that devices can't be set after installing Catalina, then do check the /Library/Preferences and if needed rename files, or even delete them, as suggested here. I think these do get reset on starting the machine up, so it should be a fairly safe process to try. In my case I didn't even need to restart the machine to recover the appropriate behaviours.