Question

the number of blades in horizontal axis wind turbines must be 3

Answer 1

Wind turbine usually have three blades as the three blades offer high rotational speed at minimum stress. 

As the more the number of blades there will be on a turbine the higher will be its torque (that is the force causing rotation) and the lower will be the rotational speed. But these wind turbines main purpose is to create electricity, so maximum speed is required in order to provide the stable and quick flow of electricity.