You can safely take the other two, if you do it now. They make these treatments stronger than needed, really, and so two days of the three-day treatment should be enough to clear it. Yeast infections are actually quite easy to get rid of, in medical terms, so you shouldn't have a problem. If money is no concern, by all means go ahead and restart the treatment with a new 3-day supply though. Try not to mix and match 1- and 3-day treatments at the same time, for the immunity reasons you mentioned above.
You can also try some home remedies for yeast infections. I've seen a lot of success with just drinking cranberry juice. The "yogurt finger method", detailed in the site below, also works really well for most women. These home treatments can be done at the same time as the antibiotics if you wish.