You are correct. Kaplan's curves are generous. However, if it makes you feel any better, AAMC3 was the lowest of my scores (and I have had scores in the area of 6-7 higher than what I scored on that exam) and I was doing the same thing you are likely doing now (that is, taking the AAMC's in order).
Part of your score drop can be attributed to unfamiliarity with the AAMC style. Your score will go up slightly just based on that alone.
As for what you can do about it, it really depends on how much time left you have to study. If you are writing in under a week, the best you can do is a combination of taking the AAMC tests (as those are your only source of such multiple choice questions) combined with massive studying of your weak areas (I almost want to say the word 'only' here but it's up to you).
If you have two to three weeks, I would recommend that, in that time, you go over through all 8 AAMC exams. Definitely look over AAMC3 to see what kind of questions you were missing and whether they can be attributed to content review, unfamiliarity with the style or both. That and do a combination of studying (especially in PS if you claim you are weak as AAMC is easier than the real thing, sorry to say... stick with Kaplan's subject tests and such as those, while hard, will prepare you well).
More than three weeks, get a hold of EK101 Verbal to boost up your verbal (as that will boost up your verbal score by proxy through familiarity) and do massive content review, making sure to understand the questions. Personally, I use Kaplan's QBank as I find it is the right difficulty for 'training' (ie. harder than the actual thing, but somewhat similar). If you take this route, be sure to uncheck the 'Verbal' box for Kaplan's QBank it is not very representative of what you will see.
In conclusion, the score drop is somewhat unusual and you will likely have to work at it to reach your targets but it is something that can be achieved through a combination of thorough (ie. concept-based) content review and increased familiarity with the testing style (though the amount of improvement depends on how much time you have remaining).