The answer to your question is an engineering one. If you look at the great majority of racing car steering wheels, it is evident that 3 is considered the minimum number of spokes that will also provide the required strength. I've attached a picture of a 917K wheel. For most sportscars, the same lightness and strength is desirable. From a marketing standpoint, a 3-spoke wheel evokes racing, if what you are building is a sportscar.
