Why do wind turbines have 3 blades?

Generating energy from the wind to do work is nothing new.  For example, 1000 year old windmills in Iran are still in use today 1. These technologies have evolved from simple machines used to pump water and crush grain to the standard 3 blade design used to generate electricity today.   Electricity is made when wind causes each rotor blade to add a force which turns a shaft that rotates a magnet past a coil of wires. 
 
Are more blades better?  More rotor blades will indeed increase the rotational force of the rotor and produce more power.  Consider though that rotor blades are one of the most expensive parts of a wind turbine, making up 20% of its material cost 2.  A 2.5MW wind turbine can cost 2 million dollars.  Adding another heavy blade would also require costly structural upgrades. A rotor for a typical wind turbine (model SWT2.3-108) weighs 60 tons and each blade is 53 meters long.  Further, more blades do not add more power in equal proportion.  Two blades are 10% more efficient than a single blade and three blades are only 5% more efficient than two blades.  Four blades will add an even smaller marginal gain in efficient but at a huge cost.  The benefit to cost analysis will show that the extra weight, money and diminishing performance is not economically worth it 3.  PS: The reasons why we use a 3 phase electrical system vs more or less phases is for similar economic considerations and law of diminishing returns.
 
Are less blades better?  Two bladed designs can match the performance of a 3 bladed design in two ways: increase the blade width by 50% or increase the rotation speed by 22%.  Increasing the width also increases its costs and weight which defeats the purpose of only having two blades in the first place!   Increasing its speed creates more noise and centrifugal force which also adds apparent weight.  Some wind turbine rotors spin on average 15rpm.  The tip speed can reach 190mph 4.  This video shows what happens to the blades if they spin faster than they can structurally support.