[hotpotamus]

My understanding is that tri-engine jetliners were actually more to fulfill a regulation than for engineering reasons. There are regulatory limits to how far a plane is allowed to fly on one engine called ETOPS - I forget what it really stands for, but the colloquial expression is: Engines Turn Or Passengers Swim. It used to be limited to around 2 hours I believe, meaning you've got a bit of time to divert and make an emergency landing. But if you're crossing the ocean, it's not really possible, so for a long time if you were crossing an ocean, you needed a 3 or 4 engined plane to do it.

Jet engines are extremely reliable however, they fail on the order of several hundred times less often than piston engines, and are very well proven and have basically only improved, and so ETOPS rules have been relaxed quite a bit meaning that a lot more ocean crossing routes are available to twinjets.

[robocat]

Plus having more engines doesn’t help if failures are correlated, such as some fuel issues, or http://news.bbc.co.uk/2/hi/uk_news/magazine/8622099.stm

[jabl]

Yes, ETOPS ratings have been extended as engines have gotten more reliable.

Another factor is that we have learned to make bigger engines as well. So nowadays with two engines you can power a pretty big plane. And due to how turbine efficiency tends to scale with size, two big engines is more fuel efficient as well as saves on maintenance costs vs. having more but smaller engines.

For an extreme example, look at the B-52 with 8 engines. That was what was available back when the plane was designed, but nowadays the thrust from those 8 engines (about 600kN in total) can easily be exceeded by two modern large turbofans.

[coredog64]

Extended Twin Operations.