nono, as part of the magic-item-creation rules, there is the whole "charges per day" thing. And as an addendum to that part, you can have fewer than one charge per day. There are several magic items priced this way (off the top of my head, I can't think of any, but I know I've seen them. Heck, there are items in the Sunless Citadel and Deep Horizon that have this feature).
So, if you want one charge every three days, then you divide the price by three. Ta-da!
