The ever-increasing reliability of modern jet engines means that virtually all new designs going forward will be twins. And for those few applications where additional redundancy may be advisable, as Gigneil pointed out, it is easier to simply hang another pair of engines on the wings (a la A340) than to (a) design and build the complex tail structure required to house a third engine on the centerline, and then (b) service the tail-mounted engine.
And when it comes to prop planes...well, I don't expect to see a Trislander lookalike anytime soon.
Common sense is not so common. -Voltaire