they did not really want to design a trijet
I think both Lockheed and Douglas would have preferred twins if technology and opinion had been fully on their side.
A fin mounted engine adds weight and asks for strength where it is rather unwelcome. This was the view of my father who left de Havilland for Douglas and Douglas for Airbus.
He was a severe critic of Airbus wing design, the A330 losing out on advantages in wing/engine/fuselage integration for the A340 to have four pods. Then it emerged that the physics had been imperfectly understood requiring redesign effort anyway.
Airbus have hardly covered themselves with glory in their A340-500/600 range with its larger wing, different sweep. Once in service there has had to be some unexpected tinkering with c of g and flight dynamics but to be fair you can't always get it right first time. However talk of "optimisation" is spin doctor speak. My father railed at his and others inability to get the speed, cleanliness and economy that was desired. So someone somewhere coined the phrase "optimisation" and Leahy had his slogan.
Whereas most of Boeing's trials and tribulations have been of the "sex scandal and shopping" variety Airbus have an irritating knack of snatching defeat from the jaws of victory lately. Whether right or wrong the large twin seems to have inevitableness written all over it and in this context the trijet was a stopgap response, a page in history that has been turned.
But Sir Isaac Newton tells us why an apple falls down from the sky and by this law it seems quite fair, a plane without engines won't stay up in the air. If two part company, three's not a crowd but surely these days four is a bore.
I'll declare an interest. I work for a certain company but it might not be the one that you think!