The difference between Monistat 3 and Monistat 7 is how much medicine is in each dose. The Monistat 3 has about double the medicine that the Monistat 7 has, which is why you only use it for 3 days as opposed to 7. A seven day course at a lower dose would make for less chance of absorbing harmful levels of it.
i was also told i had to use the 7 day instead of the 3 day. its a pain, i would much rather do the 3 then the 7 but i guess the 3 day is too much medicine at one time for the baby. they make a generic form of monistat thats the exact same thing but a lot cheaper that you can use. 17 dollars is ridiculous!