How often do you miss a putt 5 or 6 feet to the left or right? Regardless of your handicap, I'll bet, "not that many!" How often does a putt end up 5 or 6 feet long or short? Probably a lot more!
This is typical of most golfers and suggests that most three putts are due not to a poor aim or lack of ability to read the green but instead due to a lack of distance control.
In order to be more consistent on the green it is important to avoid 'deceleration' during the putting stroke. Deceleration occurs when the putter slows down (even very slightly) during the forward stroke as it approaches the golf ball. It can cause havoc with distance control. Deceleration does not always result in putts stopping short of the hole. Sometimes putts are too long and sometimes they are too short. If what we have said here describes your putting you may be decelerating during the stroke.
In order to avoid this dreaded malady make a conscious effort to follow through on every putt and not end the stroke once the ball is contacted. In order to ensure that you do not decelerate when putting I use a technique that I have found to be very successful. Every time that you putt, make sure that the follow through (distance the putter travels after impact) is twice the length of the back swing.
By developing this habit you can virtually eliminate any deceleration and become a more consistent distance putter.