Why fans have three wings?

Actually some have more than 3. Three is the minimum to keep things balanced. With 1 or 2 blades, the fan will tend to wobbel depending on geometry and material used. IE there are 1 and 2 foam blades used on hand held sprtitzer devices that tourists carry around. Since the weight is minimal and cost is essential, it works. However for home ceiling fans, its important to have 3-5 blades. Generally the 5 blade fans have longer blades and are used for large rooms that nee more air pushed.