Want to scratch a Macchi C202 to suit my 100cc engine, have any suggestion on what scale I can go up to and be able to use a 3 blade prop. ?

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.

I fly a FiberClassics Spitfire on a Quadra 100 with a 3 blade prop. It flies fine. I use a Mejlik 24-10 currently, but may try a 24-12 soon enough. Looks good on the ground. Its a 102" plane. Prop is 2" shy of "Scale" I think.

I don't know the dia of the Macchi prop, but assuming 10-11', at 1/4 scale that's 30-33"; @ 1:4.5 scale (roughly what your Spit is), it's 26-28"

1:4 scale C202 would mean a 29in prop. 1:4,5 would be 25,8in