If you find it hard to read putts then you will always struggle to become anything better than average at it. You can however ensure 3 putts are eradicated by finding a formula for pace.
I have always used the the well known formula;
Championship Greens - distance +/- elevation divided by 1.4
Tournament Greens - distance +/- elevation divided by 1.3
Very Fast Greens - distance +/- elevation divided by 1.2
and so on.
When facing a severe downhill putt I take a bit more off the pace, trial and error eventually gets that right. Once you have pace mastered you can then concentrate more on the break. Again trial and error will eventually get you results, the problem being that it is very difficult to remember where you aimed the previous time, particularly if you play infrequently or across all courses.
When trying to qualify for the US Open I really struggled with the greens on Congressional. In the end I took extreme measures to get that qualifying score. It took patience and a lot of playing to build up enough shots of what I did for a solution.
Prior to every putt I lined it up and took a screenshot. I then putted. If I holed it I saved the screenshot without any alteration. If I missed, I took a mental note of where I thought I should have aimed and then drew an arrow onto the saved picture. Like below;
I then had something to refer to during future rounds. It may appear to be something that would take an age to build up enough shots on every green to be effective but it's amazing how often you put your ball in the same area. You may not have the exact distance as previously but it is easier to make adjustments when you have the correct starting point on the line.
Not for everyone, only those who take the game seriously. You can however reduce the labour required by only doing the above for certain greens, some greens being a lot more difficult than others. I only have a library of screenshots for Congressional and Oakmont, these 2 courses being the ones I find most difficulty on the greens.