Well! I voted for the third choice. Without wading through all the on-screen evidence, My firmly held opinion is that the main power generation took place in the nacelles; each nacelle having its own dedicated generator which was responsible for the fantastic energies needed to feed a power-hungry warp engine. There is a third much smaller M/AM reactor in the secondary hull which powers the rest of the ship and is somehow critical in the operation of the other two. Put another way, reactors 1 and 2 (up in the nacelles) each generate more power but are are slaved to reactor 3 (down in the engineering hull) to operate correctly.
In addition to these three main reactors, other batteries of fusion reactors would be found here and there throughout the ship. Most notably, associated with the ship's impulse engines.
That's my opinion at any rate, and it's how the system is reflected in my on-going Enterprise