this is from an american medical grad perspective:
I did not take step three before residency per se, but I did take it 2.5 months into residency with only having an elective and ER month and no medicine ward months. So, I think this was pretty darn close to taking it as a 4th year medical student. Now having completed several medicine ward months, I don't think it would've helped for the exam at all (there are few social work/how to fill out paper work questions/scut work placement questions on the exam...yeah, i'm being a little sarcastic but not too much). Point is, I don't think what you learn on the floors as an intern would translate into better preparation for the exam and I think I would've gotten the exact same score if I had taken the exam during my fourth year of medical school versus when I took it 2.5 months into internship. That being said, if you have the option of taking it during your internship year versus if you are a medicine resident and taking it during your third year of medicine, I would definitely take it in the third year of medicine simply because by that point in time you should have read a lot of the medicine text, done practice yearly in-service exams etc. which should translate into a higher score. You do have to study for this exam, but you do not necessarily have to have been exposed to internal medicine day-to-day floor work or Gen. surgery day to day for work for that matter. again, just my opinion, and I use internal medicine as a primary example because the exam is largely internal medicine and that is what I am familiar with this year.