Trijets will not reappear, because twins can do all that they could and more. It is not just about fuel flow but maintenance. Do you want to overhaul three engines or two? Also, the access to the third engine is always tricky. Whether it is the S-shape for the 727 or the conventional for MD-10/11, they require extra gantries and lifts and so forth. With wing mounted donkeys, you slide a trailer underneath and catch it as it drops off.
Also, the a/c has to be physically strong enough to take the weight and handle the thrust or rear/tail mounted pods. When design is moving towards (or has, in some cases) composite V stab, then there really is no point in thinking about tri motors.
Most of the rest of the questions asked by TiY are inextricably linked with 100 years technological development, commercial life and national pride. Not to mention some humungous egos.
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"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different." Kurt Vonnegut, Jr.