hawk37,
The motive force to drive the fan comes from the Jet engine core, and the 3.5 exponent (approximate) relates to the jet core. If the core output, both in terms of it's own direct contribution to thrust, and the contribution in terms of power applied to the fan, is raised to this exponent, then the overall thrust output follows in similar fashion.
Yes, you are correct to a degree, it's not as high as 3.5 for a high bypass engine, but not a great deal lower than the 'straight' jet engine. (If it did, the difference between N1 speed and N2/N3 speed at higher altitudes (where the N2/N3 is operating at much higher RPMs, would be much greater - It's not).
Regards,
Old Smokey