The basic principle of stroking long putts is distance control. The goal should be to roll the ball within a three-foot radius of the hole.
Putting Within a 3-Foot Radius
Do not try to make the putt, every now and then you may get lucky and have one go in, but if you learn how to lag the putt ever so gently towards the hole, you will always avoid that dreaded three putt. Hopefully you will.
Putting Practice Drill
Now how do I achieve this? Go to the practice putting green, place some tees down around a three-foot radius of the hole, and just keep stroking putts trying to put the balls inside the tees. Do this from various locations: uphill, downhill, sidehill. You will become a much better lag putter and I can promise you, you will dramatically lower your golf scores.