KLM still operates the MD-11, so the tri-jets aren't entirely gone from the passenger jet scene. 727s and DC-10s can still be found doing cargo duty, but L-1011s are just about all relegated to the private and military scene now.

Since the 60s and 70s when those aircraft were designed and introduced, jet engines have become more fuel efficient and more powerful. Thus, the need for 3 engines dropped when the same flights could be accomplished with 2 engines with less fuel burn to boot. There was also one less engine to maintain and you know how airlines like to save money.

Just take a look at what 2 engined jets those tri-jets have been typically replaced with.

727= 737-800/900, 757-200, A320/321
L-1011= 767-400, 777-200, A330
DC-10/MD-11= 767-400, 777-200, A330

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

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