I have four of these: Walmart.com: Penn-Plax: Battery Powered Automatic Fish Feeder,: Fish
They are great, never had an issue, every Walmart I've been to sells them, and they are about ten bucks. Just need two AA batteries. But, always test it about a week before you leave, just to make sure it'[s running smoothly and its the right amount for the fish.
Anyway, agreeing with everyone else. Three days is nothing for a fish to not eat. But, I like to have one of these on hand anyway. Never hurts to always be prepared.