I am not sure which equation or handbook you used, but that is not the correct answer. The calculated power for three impellers is almost always just three times the power for one impeller. My calculation for three impellers with the fluid density and viscosity given is 0.80 hp. However, since the tank might be filled with water, even just for a test, I would always design for a density of 1,000 kg/m3, which gives a result of almost exactly 1.0 hp. In any case, even if I knew how many blades and the blade widths for the turbines, I would not size a motor at 1 hp. The minimum motor size for this application should be 1.5 hp.