No. Tri-jets were useful in the past, when engines were less powerful and more prone to failure. With the extended ETOPS range of modern flights, which comes from the increased power and reliability of single engines, they have been superseded by twinjet operations. Thus, the decline of the classic midsized trijet.
You are asking about converting larger planes to trijet operations, as we just saw trijets have been converted to twinjet. The answer is no. You would potentially save on fuel with three engine ops instead of four, but in return you would pay far more in maintenance. Since the engines would be of different designs, you've got to have different parts for the body-mounted one and the wing mounted ones, and this corresponds with more training and potentially having to have different crews for each engine type. Furthermore, servicing the third engine is much more difficult. Under the wing pylon mounts are easy to get to, remove, and service as needed. Not so for an engine mounted high up in the tail of the aircraft. You'd need specialized facilities, which again costs more money.
Lastly, some other reasons why it would not be used. Trijets are harder to fly, and require more training. Pitching up disrupts airflow to the engine, and getting caught in a deep stall is terribly difficult to recover from. You're shifting the center of gravity backwards, so new calculations have to be done for redesigning the plane. Boeing doesn't want to design a whole new plane, they've got airframes that work and prefer incremental improvements on them. Oh yeah, and passenger comfort. Trijets are much louder and cause more vibrations that engines mounted in pylons.
