Just a heads up… in case this helps the next person that tries this.
I successfully used the installer in ‘Windows XP compatability mode’ under windows 11 and it installed fine. Download the SX3 installer, unzip it into another folder, then right click the setup.exe file and choose properties. In the compatability page of the properties window, choose ‘run this program in compatability mode for:’… I chose Windows XP (service pack 2).
The setup program tried to install the old elicenser but failed, saying an elicenser was already installed. The next stage of setup installed Cubase SX3, which is great.
As far as I can see the program itself does not seem to run in compatability mode, but runs well. I can now import some old songs… not completely lossless… but at least I can re-assemble the bits and peices and get it close.