Why don't passenger aircraft use the trijet configuration anymore?
The fewer engines an aircraft has, the lower are its operating cost. Engines are the most maintenance-intensive part of an aircraft, and doing a hot section inspection on two large engines costs less than doing it on three somewhat smaller engines.

When the tri-jets were developed, no sufficiently large and powerful engines were available. They were already cheaper to operate than the four-engined aircraft, and too large to have sufficient power with just two. Now, with engines which can deliver a static thrust of more than 100,000 pounds, everything below a Jumbo Jet or an A380 can be powered with just two engines.

Another reason is increased engine reliability: With three or four engines, an engine failure is less of a problem than with just two engines per airplane. Previously, route planning had to keep a maximum distance of 60 flight minutes to a diversion airport. In 1964, this was waived for three-engined aircraft, but twins were still bound to this restriction. With more experience and better engines, this rule has been relaxed for twins gradually to 120 (1985), then 180 (1988), and now 370 (2014) minutes for the Airbus A350 XWB, and twins have become much more versatile for their airlines.

Some of the old tri-jet designs were quite popular with operators: The last Tupolev Tu-154 was built only in 2013. But besides the Dassault Falcon 900 and 7X business jets, I don't know of any trijet which is still in production.