I don't see why not really, considering you're planning taking the drug much later than Flexeril's half life extends to. But after further research, it is shown that Flexeril can have interactions with other serotonergic drugs, leading to possible serotonin syndrome. Half life consists of normally 1-3 days dependent on how much you take with a maximum of roughly about 16 days to completely excrete itself from your body. But, overall I think you should be okay given that you do in fact wait the 3 days that is said for the half life to be over.