If you are using a single port Pitot tube in a 3" pipe, you are not getting an average velocity reading, but rather only the velocity at the point of measurment. You need to take several readings across the diameter and average them into one velocity reading.
If you are using a multi port averaging Pitot tube, then you need to adjust your area factor to fit your desired readings.
Or, if you multiply your readings by .65 as a correction factor, you will get pretty close to your desired results.
John