What's the difference between 3-7 vs 5-9 of clomid (clomiphene)? thanks

None: Studies in the 80s compared pregnancy rates on Clomiphene starting on cycle days 1, 2, 3, 4, and 5 and they were basically identical. Specialists are somewhat more likely than general obs to start on day 3, but outcomes are no different.

Timing: Depending on your particular menstrual pattern, the timing of the Clomid (clomiphene) would help you achieve or add/increase the likelihood of ovulation. Ovulation generally occurs around mid-cycle for women with the traditional 28-day cycle.