Just using a 3 bladed prop won't affect the size/scale of airplane you can build. While a 3 bladded prop is not quite as effecient as a 2 blade, the difference, in a warbird is really not significant. In fact some 3 blade props on the market are more effecient that other 2 blade models. Now if you want to swing a scale diameter prop, that's another matter. It's a good bet you would end up over-proping you engine and cutting noticably into its performance. Most who want to swing a scale prop resort to some type of reduction drive. A 100cc mill is a little much for a 1/5 scale... not just for power, but to fit in the cowl. 1/4.5 scale would get you 92" ws, and full 1/4 scale would be 104" ws. Either of those should be flyable with 100cc's.