Perfect example of how a 3 engine aircraft can be useful. You can get better climb performace (since it is predicated on losing one engine) out of short fields. With your example of the 900EX, the aircraft can leave a short strip (for example HHH) and then it can probably fly inter-continental without much penalty. The 727 exploited some of that by being able to have good short field and decent high altitude performance. Now with large jets, it was economics before the twin-jets and ETOPS came into being since commercial inter-continental jets at the time we almost all 4-engine. The -10 and the Tristar eliminated one engine saving costs while still meeting the overwater EO requirements.