Head of engine development Bernhard Heil stated, "We have looked at three-cylinder engines, but there are too many compromises, such as refinement and the savings aren't that significant." Essentially, he isn't a fan and believes the efficiency gain over a small four-cylinder is not worth the compromises. That is why Mercedes-Benz will not replace four-cylinders in their sedans but will develop a three-cylinder for the front wheel drives with a hybrid drivetrain.
If Mercedes developed the i8 you can bet it would not be powered by a puny 1.5 liter three-cylinder as Berhard Heil believes the layout creates too much vibration requiring balance shafts, is noisy, and is inferior to a four-cylinder layout. Mercedes-Benz gets it.