Most people take Step III while they are in their PGY-1 year. Since your free time (read study time) is at a premium, and the you only need a PASS on this exam, most people do not spend a significant amount of time preparing for this test.
That being said, people who are in Internal Medicine and Family Medicine, usually do very well on Step III because a majority of the questions deal with the same types of patients that these primary care specialties see regularly in clinic, the emergency department and office. If you are in one of these specialties, you have likely done most of your prep by just going to work daily. If you are in a surgical specialty, you might want to do more work with something like Q-Bank/USMLEworld etc so that you can refresh your medicine skills. If you are in a surgical specialty residency and a couple of years out from medical school, you definitely want to do some significant review of primary care medicine (Q-Bank, USMLEworld, Crush USMLE Step III etc).
As for score correlation on Q-Bank and USMLEWorld, use the same percentages as for Step II. USMLE Step III has the same format with one very important exception and that is the Clincial Cases. I am not exactly sure on this one but I seem to remember that the minimum pass score for USMLE Step III is about the same as for USMLE Step II (around 182) so the same percentages should be a fairly good guideline. The good thing is that your score on USMLE Step III doesn't matter unless you fail and thus, whatever correlations you have found for Step II will likely work or exceed Step III.
You have to do nine or so, clinical cases using the software that USMLE sends when you register for the test. The single most important thing that you can do to help your score here is PRACTICE, PRACTICE and practice with that software. Also, read the critiques of the clinical cases that come with that disk.
My experience with Step III was that the clinical scenarios for the questions are shorter than those that I had on Step II. On my first day of Step III testing, I had about 9 or 10 blocks of 30-40 questions in various settings like the Emergency Department, on a general medicine ward and in a primary care office. The patients ranged from peds to geriatric. The disease gambit was nothing obscure like alpha-1-antitrypsin deficiency but fairly common things like rashes, diabetes mellitus type I and II, gallbladder disease, appendicitis, meningitis, Alzheimers Disease and other fairly common things that bring people to primary care. There is a significant amount of preventive medicine on the test too (immunizations, health maintenence).
On the second day, I had more 30-40 question blocks and the nine cases. Again, being totally comfortable with the software is of paramount importance so that you can get everything done within the alloted amount of time.
The other thing to remember is not to panic if your case ends early. This usually means that you have done enough to make a diagnosis. Just finish up any orders that you need, (remember the health maintenance stuff) and write your diagnosis.
My other piece of advice is to take Step III as soon as you can especially if you are in a surgical internship. The further away from your medical school training, the more prep and the more of a "PITA" this exam will be. I took my Step III right after ABSITE (American Board of Surgery In-Training Exam) during in my PGY-1 year as I am in General Surgery. My fellow surgical interns did much the same. It took me a couple of weekends to prepare and get used to the software.
I hope that this helps.
