My experience with 3-blades has been that you lose a lot of efficiency but also a lot of noise. For 3D I'd stick with the largest diameter you can swing without effecting engine transition.
MA's are a horrible inefficient prop to begin with but it doesn't hurt to try.
I've tried a 26x12 Mejlik 3-blade on a BME, a 12x6 APC on an OS .61 pumped, and a 11x5 MA on a ST .51 and none of them suited my style of flying.
Chris