Hi Mansoor50,
You ask a great question, but it’s difficult to give you an exact answer. The reality is that when you are dealing with a test comprised of only 31 quant questions with about 25 main topics (Exponents, Roots, Work questions, Overlapping Sets, etc.), it’s difficult to say what is likely or unlikely to appear on any given exam. Thus, there are not 3 MUST KNOW quant topics.
Consider this quote from The Art of War:
"The art of war teaches us to rely not on the likelihood of the enemy's not coming, but on our own readiness to receive him; not on the chance of his not attacking, but rather on the fact that we have made our position unassailable."
Rather than trying to game the GMAT or determine the odds of seeing certain topics, you need to prepare such that you are ready for anything that may come your way on test day. Only then will you be able to walk into the test center knowing that regardless of what question pops up on the screen, you can DOMINATE it.
That being said, when is your actual GMAT? Have you taken a practice GMAT to get a baseline score? You may find it helpful to read my article about
how to increase your GMAT quant score for more tips on dominating the GMAT.
Feel free to reach out with further questions.