Normally the three readings is advice for people doing it at home on their own machines, that may or may not be so accurate, plus its important to have wrapped the arm band round in a firm way but not over tight. By taking three readings you can then present an average to your doctor over a period of time when you next visit the surgery. Either way its giving the doctor at least some readings over a longer period to work with, they are obviously looking for very high or very low readings. Most doctors will take just one reading at the surgery when you visit, and may in their own experience deduct a percentage if they feel you are on edge due to your visit, but they also have to take into consideration your present health problems at that time, ( if any) I would think its all about doctors having past experience, and some still prefer the old type BP kit.