Three blade props are used in full scale aircraft for a number of reasons. The main one being you can have shorter blades as opposed to a two blade prop, which helps when you are swinging large props, like many WW2 warbird for example. In our modeling, however, that need is seldom the case unless we are working in scale models and such. At the small diameter and other factors in our models, the three blades loose efficiency over 2 blade props, so a 2 blade is usually preferred. Some ARF trainers come witha three bladed prop, because they can turn at a lower rpm, which makes the plane quieter, and also the big blades they use can act as an air brake a little better, which can be desireable in a training situation. So unless you are flying a scale airplane or need a smaller diameter blade for a unique configuration, it is best to use a 2 blade prop.