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Search the forums - somewhere was the tip how to squeeze more than 2 tests out of GMATprep. Take 1 full test a day.
Be sure to get familiar with DS format - say, take about 50 problems, but don't agonize over each one, just get used to the problem type. It's important, cause an easy question in a quirky format can take a lot of time to crack.
All the remaining time should be spent with SC. Kill each & every SC problem you find in the books; read the explanations on those you get wrong, make notes of idioms you didn't know. SC 1000 is good for practice, but there are no explanations, so go down on those only after you do everything in the books.
Take a break about every 20 SC problems, do a couple of RCs to keep your mind flexible.
P.S.: make sure to arrange a good sleep before the exam (and the previous night shouldn't be a 1-hour sleep as well!). No matter how well prepared you are, if your brain refuses to work at G day - you're in trouble.
I don't know what stage your preparation is at, but I would highly recommend spending the last day (even 2) just relaxing. I have seen many many reports of people that have failed to execute on test day because they were stressed, fatigued and not relaxed for the exam. I think my own case was is a good example (score about 40-50 points lower than practice on my first try). On the other hand, I have read very few reports of anyone believing that they learned anything in the last 2 days that helped them a lot on the exam.
Why do you have to take the test in three days? If you aren't ready, and can't get your money back, their either choose another day (give yourself at least a month to adequately prep) or go in for a recon mission: take the test and cancel your score. The latter is just to get your money's worth. Schools will the the "cancel" on your score report, but there are a number of ways to explain yourself.