I wouldnt write an addendum unless there were some real, demonstrable problems in the testing center. The only real difference between your score 1/2 and 3 is that you studied. You probably won't need an addendum. The pattern shows that you did some real work. If you are wait listed anywhere, it is definite supplementation material.
If you do supplement, be careful not to sound like you are giving excuses. And, do not hide the fact that you studied hard for the 3rd test (no need to tell them prep course).