ok. put it this way, less is more. more is less. less valve means more mileage. also, less horsepower since less fuel is contained in the chamber. consequently more is less. more valves means less mileage, but more horsepower. self-explanatory, since more fuel is burned thus more energy is produced.

as in the case of mercedes, 3V / cyl? well, check out its displacement and internal attributes such as supercharger or turbocharger, it is well compensated.

hope my point is taken. :D