You can download a program called ThrustHP
http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm
No matter what you use as a calculator you are just getting theoretical numbers that may or may not happen in real life.
The theoretical programs do not care about diameter when they calculate speed.

Using your example calculation:
3 inch pitch
18,000 rpm
51.136 mph

As long as it is turning 18k (not more or less) a perfect 3 inch pitch prop in a perfect world will generate the same mph regardless of diameter.

When you get into the real world everything changes. Engine horsepower and torque, prop design, air density, and the airframe affect the actual performance you get. Unfortunately no one can predict that for you.

You have to buy a stack of props and do your own comparisons in the conditions you have to deal with.