There are no hard-and-fast rules on this, but the adcoms do like to see an applicant make a real effort to improve the GMAT score, and as you know, higher scores make a difference. Taking the test multiple times shows that you're dedicated to the process and really committed to getting into a top school; taking it too many times without marked improvement could be a red flag. There's no definite number of what "too many times" is but if the scores aren't progressing upward, then you should reevaluate the approach. Hopefully with the advantage of the Veritas Prep package, you'll really nail it this time, but if not, I wouldn't necessarily say not to try a fourth time, too.
Good luck with it!
Lisa, Columbia MBA | Admissions Consultant | Elite GMAT Preparation and Admissions Consulting | http://www.VeritasPrep.com