chornedsnorkack, I think a trijet would be very heavy. The APTU envisioned would be a CFM class engine neadly fitting into the tail. It would provide the Ecoliner the required power at take-off with an engine failure while not requiring an engine to developped even larger then the GE90-115, while still providing twin cruise economics.
For some reason, it seems that airplanes want to keep the engines equal. The only exception is the fourth engine of Trident. From first principles, there would seem to be a good case for having a quad with a pair of large inboard engines and a pair of small outboard engines: this way, you do not have the critical case of an outboard engine out at the low airspeed of takeoff, and you can scale your fin and rudder to more modest asymmetric thrust torques. But for some reason, all quadjets other than Trident try to have all engines equal and scale their tail to outboard engine out - whether A380, A340, B747, B707, CV880, M119...
Do you know how exactly the weight and efficiency of engines scales with thrust? Would a MD-XX NG with, say, 3 GEnx engines be competitive with a 777-300ER with 2 Ge90-115 engines?