First, don't be too disheartened if you are good in other areas. The 3 topics that you mentioned are arguably the toughest that are tested on the GMAT.
If you feel you are comfortable with the concepts, go ahead and do focused study on each of the areas. Do a tag search on quant forum (PS and DS) and try to solve 10-15 questions in a bunch. Also, attempt problems in Bunuel's signature.
You will get a better idea of what the questions are asking and question patterns. Once you are done with a question, try to do it with a different approach. Check the official explanations for each of your correct and incorrect answers.
Practice goes a long way.
My Debrief | MBA Timeline - New! Stay on top of deadlines, receive recommendations for each stage, get reminders.