I can see one possibility.
Use three for maximum power and then two for cruise. Supply enough to the third to carry it's own weight (roughly 10%) and extend ETOPS?
Further thought, use it as or instead of an APU?
Just thinking back to what the five engined Trident might have become.
Sound alleviation sounds (sorry) like a bit of a red herring. Very hard to get high aspect ratio fins to be in the right place most of the time.