If you need to receive Step 3 scores before March 28, 2018 it is recommended that you take the exam no later than December 31, 2017.
Most Step 3 scores are reported within 4 weeks of testing. However, because of necessary modifications to the test item pool, there will be a delay in reporting for some examinees who test beginning the week of January 15, 2018. The target date for reporting Step 3 scores for most examinees testing the week of January 15 through late February will be Wednesday, March 28, 2018.
Examinees should read the full announcement on the USMLE website.