just as a general note, the free 150 isn't the greatest in terms of being predictive of exam content. the questions are shorter and are much more simple recall than the USMLE seems to be spitting out these days. however, people have reported that their score on the free 150 ended up being pretty close to their score on the real deal, but most of the cases i've seen were where people took the free questions just a few days before their exam (as opposed to 3 weeks before).
are you doing any Q-banks (e.g. Kaplan, UWorld)? are your scores improving on those? i definitely think you can improve in a few weeks, as long as you focus on boosting up your weaknesses. incorporating questions is also key. make a plan to start slow (maybe 30-50 questions a day) and work your way up to 2-3 sets of 48 questions per day. and read ALL explanations (even the ones you got wrong).
if you want a better score estimate, or at least something for comparison, you might want to try one of the NBME self-assessments ($45 each), or you can do a kaplan / UWorld self-assessment. i think the kaplan assessment comes included in the price of QBank, whereas the UWorld assessment is an additional cost. browse the forums here for advice on which ones of these people have taken and how they've done.
