Imagine a rotor with 3 bearings from a gasturbine, the outer system in a 2 shaft machine.
IRL in field you can only access the end planes and only with trouble the hot end so in the field if needed it is mostly balanced in the cold end with kind of fingers crossed.
So if we now have capacity to create a balancing setup with 3 pedestals (soft) and somehow create a plane in the middle is it really needed, part from supporting the rotor? Middle bearing IRL in the machine is a relative bearing together with the inner rotor and by that maybe reduced stiffness but major rotor weight is also in the middle. So you should in principle run a 2-plane and press static/couple calc button and fix the static by balancing in the middle. Then adjust the remaining coupled in the ends. I have done that with Archimedes screws and such with very good results. I have seen magic to be done at half critical speed with a middle plane on papermachine cylinders but that supposedly "only works in stiff (balancing)machines". Having a bearing of sort in the middle do influence the rotor shape naturally. These machines are old, really old of my fathers vintage and still run like never ending and the OEM is not so fascinated in keeping them alive...... So a procedure like this may never been tried but if it works having the rotor assembled so there are minimal work on site would be great. I think aero rotors are run like that before final assembly but I never saw a 3 pedestal solution part from balancing machines for drive shafts for trucks and cars.