2D TVC only gets you pitch axis control, which can also be done with control surfaces. tailless designs have yaw axis problems.
I believe the trijet will significantly help yaw axis control with differential thrust. the advantage over 2 engines is the fine tuning of yaw control without compromising overall thrust with differential thrust control. as a thought experiment:
let's say you can either have 2 new, very large engines that max out at 60/60 or 3 existing engines that max out at 50/50/50, each running in cruise conditions at 50/50 or 40/40/40. Nice round numbers for estimation.
2 engine maneuver with differential thrust: redistribute engines from 50/50 to 60/40 which gets to the limit of the engine or 50/40 which reduces total thrust. You cannot maneuver with differential thrust without stressing the engine or reducing total thrust.
3 engine maneuver with differential thrust: redistribute engines from 40/40/40 to 35/40/45. same total thrust, does not stress any engine, but maneuvers with differential thrust. And if you cut cruise to 30/30/30, you have even more room to maneuver with the centerline engine making up total thrust that the maneuvering engines don't.
