Bad idea to count on it? Yes. Bad idea to do it if it happens to post in time (but not count on it)? No.

I have no experience with this kind of promo at Marriott. I do know that in several other hotel programs where I've participated in these kind of stay X times, get a free night certificate promo, the free night certificate took longer to post than the stays themselves took to post (ie, the free night cert did not post at the same instant the stay posted). If Marrriott does this promo the same way, three days would most likely not be enough.

Again, while not knowing how it works specifically at Marriott, based on other programs, if the hotel where you want to use it lets you book a free night on points within hours of check-in, then you may be able to use a free night certificate just as close to check-in. But if it has capacity controls on regular points stays, then I would expect at least similar capacity controls on the free night cert. So it may depend on the particular hotel more than just the certificate.