The DC-10 was a rushed programme to avoid Douglas being frozen out of the 1970s widebody market by Lockheed and Boeing.

Similarly, the MD-11 was a cost-restricted update of the airframe to avoid McD being frozen out of the 1990s widebody market by Airbus and Boeing.

McD management wouldn't fund the more ambitious four-engined MD-12, so the trijet's fuselage was stretched and aerodynamic tweaks applied.

The MD-11 never met its performance targets and heralded the end of the Douglas commercial line. It was fairly quickly relegated from pax to cargo service where it has a good payload but little else to commend it.

I always thought that the third engine was thrown into the tail of the DC-10 when they learned of the L-1011. However, I think I first heard that from a retired Tristar pilot.

https://en.wikipedia.org/wiki/Lockheed_L-1011_TriStar