Considering the ETOPS rule, the trijet design is close to death in the commercial passenger aircraft sector.
A Boeing engineer once said "Three engines? That's the optimum worst!"
As early as in the early 1960s, Boeing engineers had to make a kind of roadshow with airlines in order to be sure the customers would order a three-engine aircraft. Eventually they went for it because this added some reliability to the aircraft, and therefore improved the aircraft use.
In 1978, when Boeing proposed the 777-100 and the 777-200, two versions of the 767 with three engines (respectively for transcontinental and intercontinental flights). But that's the last time they proposed a three-engine aircraft AFAIK. Even later, after 1986 when Boeing made quite a few proposals as part of the 767-X development program, the idea of a three-jet didn't come up.
ETOPS really killed three-engined aircraft. First the 60-minute rule applied to all but four-engine aircraft. Then this was restrictied to twins, giving an advantage to the tri-jets. But then with the introduction of the ETOPS rule in the early 1980s (first ETOPS flight in 1984), this was the end of this advantage.
Twins have always been more economic than tri-jets. But tri-jets were more reliable because engines were not as reliable as they are today. Now with the great improvement of the engine safety (they are among the safest pieces of an airliner), there's no argument remaining for a tri-jet.