[inferiorhuman]

In the case of the quad jets, Boeing tried the 747-SP and had minimal marketing success.

In the case of the trijets the MD-11 lived on as a freighter because it had a much higher capacity than anything else smaller than a 747.

  It was never designed to be used anywhere else as a
  general design. Two engines did that better.

Not quite. Dassault still makes a three engined bizjet and in theory the Chinese fly a three engined stealth jet.

[MBCook]

I didn’t know there was a three engine business jet, my knowledge is mostly passenger airliners and even then just from an amateur perspective.

Other than being able to identify a couple of famous ones I don’t know a ton about military airplanes either.

Thanks!