Why do ceiling fans have three blades?

Irrespective of the number of blades that a fan has, the inter-blade radial sectoral angles are always the same. In other words, the radial sectoral angle is 360/n degrees where n is the number of blades. For example, if the number of blades is 3, then the sectoral inter-blade angle is 360/3=120 degrees and when the number is 4 the angle is 90 degrees. This condition of similarity of inter-blade angles, ensures a zero net effective centrifugal force on the blades and thereby, avoids wobbling of the fan during its running.

Now let us know why we find 3-blade ceiling fans most often. We use any ceiling fan as a device to circulate air which in turn hastens evaporation of sweat leaving a sense of comfort to us. The circulation of air is caused when the fan blades chop the air in front of their leading edges and push the air downwards via their lagging edges. This is achieved by the slight bending each blade has along its length such that the leading edge is slightly above the lagging edge and by a gradually varying (usually decreasing) width it features along its stretch from the motor.