We all know people who got in on the second (or third) try. It doesn't happen magically and it doesn't always happen in the next cycle. Those who are really serious about getting into medical school make a serious assessment of their weaknesses, the aspects of the application that were sub-optimal, and make a plan to address the weakness and improve the aspects of the application that can be improved (earlier submission, better targeting of application to schools). Not every reapplicant is successful in gaining admission but for some it is well worth a second attempt.
What percentage of applicants apply for three cycles? I don't know but I can assure you the number is very small.