I think the extra reciprocal mass would not only imbalance the engine, but also knock down the overall performance of the regular 4 cylinder.
There used to be lots of 3 cylinder cars in the USA. The previously mentioned Metro, the Daihatsu Charade, the Subaru Justy...
I don't know why a Suzuki G10 motor in a Chevrolet Metro sounds like crap and the Triumph 3-cyl sounds so beautiful.
Honda built a lot of weird V configured engines in the '80's.
V2s, V3s, V4s, V5s, V7s...etc...
They had an oval pistoned 32 valve, 8 sparkplug, V-4.
Their V-3s were very successful in Moto-GP.
And the next VFR is supposed to be a V-5
http://www.motorcyclenews.com/upload...ug-6-front.jpg