IN terms off efficiency I am sure that 2 high power engines are more efficient than 3 mediium power engines ( a lot of extra weight to carry around ). It would be interesting to know why they chose 3 engines in the first place. I thnk the Chinook with only 2 engines has reasonable high altitude performance. Interesting the the CH53E has 3 engines ( 3 *7500) vs (3*2312) for the EH101. I wonder what the 101 performance would be with 2 Chinook engines in place ( 2*3750 )