The most common three cylinder crankshaft looks like one half of an inline 6 cylinder crank, three throws, each 240 degrees apart. Most recent automotive 3 cylinders have balance shafts. Ford introduced one about a decade ago that did not have a balance shaft, but the ancillaries were carefully placed, along with the crank and flywheel, to mitigate vibration. Parallel twins, which you were asking about before, have a variety of vibration issues, that in current motorcycle and automotive practice (FIAT), are mitigated with balance shafts. Air cooled twin cylinder motorcycles of the 1960-70s, could be divided roughly into two large camps, British 360 degree cranks where the pistons rose and fell together, "pop, wheeze, pop, wheeze,". Designed almost a generation later, Japanese motorcycles had 180 degree cranks where the pistons alternated rising and fallin. This led to a rocking couple, one end then the other of the engine rising and falling. Among the Japanese 4 strokes, the sound would be pop, pop, wheeze, wheeze.I am such a fan of three engines maybe three cylinders is just a natural progression? So are three cylinders pop, wheeze, pop, wheeze, pop, wheeze?