A couple of points.....
Firstly, EPR in a high speed descent (in my experience) will only get down to 0.97.... Not 0.9, that's REALLY low!!!
And secondly, I believe that the economics of the three spool engine (in addition to the above) come from the fact that more compressor stages are working at optimum RPM with the addition of the intermediate pressure turbine...
BN2A
