I don't know offhand what the school averages are for the schools you are targeting. However, if the GMATs are significantly below the average (i.e. more than 20 points below the average), then it'll be worth taking it a 4th time - better to have a stronger score on the 4th attempt than what you have now if it's below the average. If you're already at the average with your highest score (640), then don't bother retaking it.
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