WHY DO WE NEED THREE SEMICIRCULAR CANALS IN EACH EAR,RATHER THA ONLY ONE?
This is a very interesting question; I had no idea what the answer is until I looked it up. See http://en.wikipedia.org/wiki/Semicircular_canal
Apparently the three fluid-filled canals have nothing to do with hearing. They function as an accelerometer to detect small motion and help maintain balance. You need three of them because there are three dimensions: up-down, left-right and forward-back. The mechanism by which acceleration is sensed (uneven pressure against hair-like cilia connected by nerves to the brain)is similar to the way sound is detected. It is logical that organs that work similarly, but with different functions, would be located in the ear.