For a given motor, a 3 blade prop is generally a bit shorter than the 2 blade (1 inch being the basic rule of thumb with models). Which could allow the motor to be mounted a bit lower in the boat. Could help with the overall balance and performance of the thing as air boats can be quite tippy at speed.
Also, flipping a prop over does not reverse it. The only thing it will do is reduce the performance of the prop because you now have it on backwards. The prop blade is like a wing in that it has an airfoil with a distinct front and back (top and bottom for a wing). The front, top, curved side should always face forward.
There are 2 basic prop types, Tractor and pusher. So named for the configuration of prop/motor in the airframe. Basically, motor behind prop is a tractor. Motor in front of prop it's a pusher. If you compare a tractor and a pusher prop to each other, the only real difference you will find is the direction of the twist in the blades. They will be opposite.
With electrics there is no real need to have a dedicated pusher prop. Use a tractor and simply reverse the motor rotational direction. On a brushless (3 wires) motor simply swap any 2 motor wires. So instead of the 3 wires connecting to the ESC as:
1-1
2-2
3-3
Make it something like:
1-3
2-2
3-1
Motor power will remain the same.