First off, every golfer will 3 putt, but in your case you indicate it is happening often and it's
good that you have recognized this.
I suggest first practice short straight putts (around 5-7 ft) to build your confidence.
A good putt will end up within 6 inches of the hole if not made.
You can practice indoor or outdoor, with the use of several dimes.
Lay the dimes about two feet increments on the line of the putt for visual targets.
I like using a laser light indoors which give me a good visual of the ball rolling towards the target.
Then after you gain confidence, you should learn to control the speed/distance of the putt.
Again, practice (5-7 ft) around the entire hole on a putting green with slope/break.
Then practice longer putts (25ft plus) and try to control the distance where you are achieving the putts
to stop about 5 feet or less from the target.
My goal is to have putts stop less than 2 feet past the hole on every putt if I do not make them.