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Engine longevity depends more on how robust the construction is rather than the number of cylinders.
There have been some strange engines over the years... Early Ferraris used 1.6 and 2.0 V12s... And there are big industrial diesels with three, two and even single cylinder layouts.
For modern engines, not only do fewer cylinders mean less friction, they also allow you to more easily package multiple valves, dual spark plugs and/or direct injection hardware. Going down to three cylinders makes it more feasible to make small diesels with direct injection. Vibration may cause longevity issues, or they may not. Four bangers aren't well balanced either, without balancer shafts or DMFs, but they tend to last a long time, anyway.
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