Regarding the DC10 and L1011 I recall they grew out of an American Airlines requirement for a 250 seater that could operate non-stop across the USA off La Guardia's runways and AA envisaged a big twin. Both Douglas and Lockheed looked at future growth options and hedged their bets by going for 3 engines. Douglas did look into a twin version of the DC10 later but it was not economically viable and was dropped.
AA supposedly said some years later that the A300 was what they were after, the DC10 being too big, but it came along a few years too late although AA did eventually buy some A300-600's.
I can remember Boeing stating at a presentation back in the late 80's/early 90's that no-one would build a 3 engined aircraft again as it was not economical. In their opinion all new conventional designs would be either twin or quad. The MD11 was an exception as it was a derivative and could be cost justified. Airbus said more or less the same thing.
Brain Fade,
Yes, the A330 and A340 were designed as a common airframe. The system architecture is common and the wing aerodynamics are the same. They are built on a common assembly line and share a single serial number sequence ie. the first A330 was MSN 12 the previous 11 being A340\'s.
The A340 was supposed to be the long range model and A330 the intermediate range one however there became a demand for a longer range A330 so the A330-200 appeared.