Metro man:
DC10 and L1011 came into being because of an FAA rule exempting aircraft with four piston or three turbine engines from ETOPS requirements.
Not exactly. Four donks across the pond was the gold standard from the days of flying boats and DC-4's, four DECADES before ETOPS was ever thought of.
The trijets were readily accepted by regulators because of proven reliability of turbines in the 70s. Canadian operator Wardair ran scheduled flights Vancouver-Hawaii in a 727. Twins were restricted to 90 minutes overwater IIRC. It was Airbus' A300s and the like that wanted to "push the envelope" to operate twins further, and for this, the ETOPS (i.e. "Extended") rules were formulated in early 80s. Existing aircraft had to be modified and/or specially tested or inspected to meet ETOPS approval.
By about 1981-82, with the 757/767 in the offing, the push was on for ETOPS approval "out of the box", on initial delivery of new aircraft.
Since then regulators have been looking closer at 3- and 4-engine birds, recognizing that cargo fire suppression and other airframe-related systems are potential risks as much as engines, and are pushing to incorporate ETOPS-style thinking into 747s and A340s.