I had thought about that too at one stage, thinking why not just ace Step 3 to just further improve the app? But I had also read somewhere (as I can vaguely remember) that the vast majority of residency programs either don't want the applicant to have already sat Step 3 prior to matching, or in fact there are licensing reasons against sitting it pre-residency as well (something like only 5 states allow for Step 3 to have been taken pre-match).
The fact of the matter is, your previous step scores best predict your future ones. No AMG has Step 3 going into his or her match, and because the AMGs overall perform better on all of the other steps, a "high" Step 3 wouldn't mean anything coming from an IMG.