The GENERAL rule of thumb is: When going from a 2 blade prop to a 3 blade prop, either go down 1" in diameter and keep the pitch the same (for instance, go from an 11x6 2 blade prop to a 10x6 3 blade prop ) OR keep the diameter the same and go down 1" on the pitch (for instance gro from an 11x6 two blade prop to a 11x5 3 blade prop ).

Your choices, to a great extent, will be determined by what size 2 blade you are now using and what sizes/pitch are available in a 3 blade prop. A lower pitch will provide more thrust (get you "out of the hole" faster and provide a slightly faster engine response time ), but knock down the top end, while a higher pitch will give you more top end at the expense of your "hole shot".

In any case, be advised that a 3 blade prop is substantially more expensive than a 2 blade prop.

Hope this is of some help.