As far as I know, you can use a review that is up to three months old. I know I've hung onto a review for a little while because I unintentionally wrote too many SPEW reviews for one month or I knew I was going to be uber busy in the next. I don't see why you couldn't. I would find the rule, but I suck at rules.
But you could submit those extra reviews for next month, right? Is that okay? I have a SPEW Buddy thing going on, but I already submitted all three reviews. . . I wonder if we could submit the extra ones for next month.