Why pulse is felt using middle three fingers?

Yes pulse is felt using three fingers only the siddha doctors and acu specialists.

The pulses are felt superficially and also at deep level.

On the left hand the superficial pulse felt by fore finger is for Small Intestine and the deeper one is for the Heart. Same manner with middle finger superficial is for Gall Bladder and deep one is for Liver. With the ring finger, the superficial is for Urinary Bladder and that with the deep one is for Kidney.

Same way on the right hand with the same order the organs could be studied are:

Large Intestine and Lungs

Stomach and Spleen

Triple warmer and pericardium