is it worth investigating the option to move the physical machine running C3 into a virtualised host on C4/5/6 ?
How would that help? The problem being addressed is that the software will no longer be maintained, not that the host hardware is about to crash and burn.
If you have applications that only run on C3, then it provides the information necessary for maintaining future C3 installations on newer hardware that C3 does not natively support. I would think it would be sufficient to state that C3 will run inside a virtual container and that you would follow X existing document to setup virtual containers and Y existing document for installing C3. pps - C3 is no longer maintained, migrating to C4/5/6 is recommended.