3 cylinders are naturally balanced. The C1 engine is very smooth.
I don't think that's strictly correct. A 3 cylinder engine is not particularly well balanced at all, but may seem so as nearly all modern ones have a "balancer shaft" driven from the engine which cancels out the inherent out-of-balanceness of the 3 cylinder arrangement. Thus, BMW's "brick" motorcycles came in 750 3 cylinder and 1000cc 4 cylinder versions. The 3 cylinder had a balancer shaft, and as a result was smoother than the already-balanced 4 cylinder which didn't need a balancer shaft.
My Triumph motorcycle is a triple, with balancer shaft, and is lovely.
But an in-line four is naturally balanced, but not as well as a straight six, whilst a triple is not.
My four cylinder Saab has a balancer shaft as well, and is very smooth indeed.
I've previously owned a straight six, and it was super smooth - no balancer shafts and a fairly unsophisticated engine, so it was inherent in the layout