TL,DR: three blades (1) are a good balance of torque against drag at different speeds, (2) help reduce unbalanced forces compared to two-blade turbines, and (3) provide visual balance, smoother motion, and quieter operation to improve community acceptance.
Is it related to automobile wheels always seeming to have odd-numbered spokes?
There are plenty of six spoke wheels?
TL,DR: three blades (1) are a good balance of torque against drag at different speeds, (2) help reduce unbalanced forces compared to two-blade turbines, and (3) provide visual balance, smoother motion, and quieter operation to improve community acceptance.