five goldfish in a 10gallon tank is extremely overstocked- Goldfish need 20gallons for the first fish and 10gallons for every goldfish added after that. I highly doubt they'll be okay for three weeks without constant water changes. You might want to look into bringing them back to a LFS. I'm sorry if it sounds harsh, but there's just no way they'll be okay for that long in a 10gallon tank without water changes, even with water changes, it's way too overstocked.