Putting it close from long range depends on two things being achieved. A correct read of the green followed by a good stroke with the ball being struck on the sweetspot of the putter.
This drill from TG Elite Pro Chris Ryan will test both of these factors on the practice green. Give yourself a long putt and read the break. Then, form a gate with two tees no more than six feet in front of the ball.
The gate should be positioned so that the ball needs to travel through it to finish close to the hole. It will give you great feedback on your stroke (if you miss the gate, your stroke was clearly off) and your green-reading skills (if your ball goes through the gate but finishes far from the hole, you have mis-read the break.)
Give this drill a try and your long-range putting should improve considerably, helping you prevent the dreaded three putt.